Backliners
The mainstays here are Staff Elementalists, Necromancers, and Mesmers. A balanced 25-man zerg should ideally contain 2-4 Staff Elementalists, 2-3 Mesmers, and upwards of 3 Necromancers; if you're going to overload one backline class, Necromancers are the most desirable to have in surplus. Note that the number of Staff Elementalists you want doesn't really scale proportionally to the size of the force--even a 15-man group will still need 2-3, and even a 40-man group still doesn't really need more than 4--because there is no limit to how many allies can blast their Combo Fields.
In short, your back line stays 600-1200 range away from the front line and supports your front line and pressures the enemy front line. In practice, it's a bit more complicated than that. Traditional MMO class roles have implanted certain stereotypes in our minds that the light armour classes should be standing at the back doing all the DPS while the heavy armour classes stand at the front and take all the damage, but GW2 breaks that mould to some extent. Certainly DPS is an important part of the back line's job, and like everyone in the zerg, you should be trying to contribute as much DPS as you can, but DPS is only one part of your job. A huge part of your job as a backliner is support and control. While the frontliners are all tangled up wailing away, you play the cards that determine which frontline wins the scrum.
Backliners are more fragile, but with the absence of true "tanks", they can't afford to be too fragile. You don't want to be in a full glass build that melts when touched, because you
will be touched. Proper positioning will help to reduce the frequency with which you get touched, but no matter how clever your positional play, it's still going to happen. Another important part of backline survival is having tools to escape when that pressure inevitably does come. As far as that positional play is concerned, a lot of it comes from experience, and details go beyond the scope of this post, but some general guidelines include trying to find a different plane from the frontliners--above is best, but below can sometimes work too--and thinking of yourself as a satellite orbiting around the frontline--work with the full 360 degrees.
Staff Elementalist
Build Framework
Despite the stereotypes that you might have of elementalists from other games and even other gameplay areas in GW2, your primary function is
not DPS. You are the master of Combo Fields. You have ready access to all four of the most important Combo Fields--Water, Ice, Fire, Air--and your most important job is to be delivering the right combo field to the right location at the right time. Playing a Staff Elementalist well is very tactical; in addition to the general backline issue of positioning, you have to read the flow of the battle to make decisions, and you have to coordinate with the rest of the zerg. Perhaps more than any other class, being in Teamspeak is indespensible, as all those "short cooldown Blast Finishers" I've mentioned in other builds don't count for much if people don't know when to use them; always call your Combo Fields if possible.
The 30 points in Water and 20 points in Arcane are a good stepping off point for an Elementalist build, leaving lots of options for the remaining 20 points. The 30 Water not only helps to address the glassiness of the class a bit directly through the Vitality, but you've got awesome Major Trait options throughout this tree. You've got a lot of sources of Regeneration, so you'll get a lot of mileage out of Cleansing Water. Though your healing contribution is vital, the vast majority of it is coming in the form of your Water Fields which other players are blasting, and so you're not doing the healing directly, thus the new Aquatic Benevolence trait is not nearly as powerful as it might seem at first glance.
Cantrips are your escapes, and you will need them. Mist Form does everything that Stability does whilst also making you invulnerable and giving you out of combat movement speed while in combat. It is a must have. Lightning Flash is also incredibly useful for repositioning yourself. When harassed, you can often blink to the far side of the front line scrum, making it impractical for your assailant to pursue you. Note that, although not a Stunbreak, it is instant cast, so you
can use Lightning Flash while stunned. You have several options for your third utility skill. Frost Bow's #4 skill does a ton of damage, Arcane Wave gives you a great Blast Finisher, and Armour of Earth is another Cantrip with excellent boons. Ether Renewal is one of the best heals in the game, not only healing decently, but curing up to eight conditions on a fifteen second cooldown. Your choices of Elite are all a bit underwhelming in large scale WvW, but each can be useful. Popping Tornado immediately after casting Meteor Shower greatly increases its damage, and the knockback can be situationally useful. The Water Elemental does drop Water Fields and survives reasonably well. The Fiery Greatsword can do a lot of DPS in the hands of someone who can survive using it in the middle of the frontline scrum, and it's also fantastic for very substantial PvDoor DPS.
On Swap Sigils work brilliantly for Elementalists, because they are triggered by Attunement Swapping, and you should be cycling through Attunements quite regularly. Coupled with Runes of Strength, a Sigil of Battle should consistently maintain 9+ stacks of Might on you, which is worth an approximately 15% increase in DPS.
2/2/0/6/4
Between the Runes and Sigil coupled with Internal Fire and Bolt to the Heart, you have most of the strongest damage modifiers incorporated into the build. Vital Striking, the Adept trait in Water, offers a further minor damage modifier, but this can be substituted for Cantrip Mastery for more survival cooldowns if you feel the need. If you find yourself needing more survivability, you can also consider Soothing Disruption in place of Aquamancer's Alacrity, giving you more dodges from Vigour and more condition removal along with the healing from Regeneration, but the longer Water cooldowns is a significant hit to your ability to perform your critical tasks. Alternatively, you can take a hit to your DPS to shift 10 points from Fire or Air into Earth. Shifting 10 points into Arcane for Evasive Arcana, which gives condition removal from Water or a Blast Finisher from Earth is also an option, but because of the nature of being at range when using staff, Evasive Arcana isn't as practical as it is for Dagger/Dagger Elementalists.
As a general rule, you're going to want around 2200-2500 Armour on a Staff Elementalist. You're slippery enough with escape skills to go fairly glassy, but you do need enough defense to withstand some pressure while you attend to your key roles.
Note: Eruption (Earth 2 on Staff) applies its Blast Finisher at the
end of the skill, so if you want to blast your own Combo Field, you can do this by starting in Earth, casting Eruption, then swapping Attunements and dropping the Combo Field on top of the Eruption. Note also that you can swap Attunements while casting another spell.
Necromancer
Build Framework
Necromancers present a potential exception to the mantra of "ignore anything but Soldier and Berserker". You can still build perfectly viable Soldier/Berserker Necromancers, but in part because they have the highest HP coupled with the lowest Armour, and in part because of the function of certain traits, Knight stats can be very useful on a Necromancer. Necromancers are in many ways the true glass cannons of WvW. While their high HP pools and Death Shroud may make Necromancers appear to be fairly durable, they are still light armoured, and they have far less to work with in terms of condition removal and mobility than do Elementalists. As such, Necromancers generally need to build a bit more tanky than other backliners.
Some of that tankiness comes in the form of Life Steal from Vampiric and Vampiric Precision in the Blood Magic tree. Vampiric Precision is another reason to favour Knight's gear, as the Precision generates more crits which in turn generate more healing. Whereas an Elementalist does most of their DPS and support through their Staff skills and can use their utilities for survivability, much of a Necromancer's DPS and support comes from utility skills. Chief among these are the Wells, and for these we take Focused Rituals from Curses and Ritual Mastery from Blood Magic.
The choice of Staff is an obvious one; Marks offer significant AE damage along with useful control effects and a Blast Finisher. The decision to take a mainhand Dagger will be a bit controversial, but it is simply too strong to leave out; many will question the omission of Axe, but the simple fact is that you'll do more damage auto attacking with the Staff than you will with the Axe--they have identical damage, but the Staff pierces. Dagger is very much a niche weapon, but it is the only choice that offers something clearly different from just sitting in Staff. Mainhand Dagger is a single-target melee range weapon, which seems like a strange choice for a backliner, but it deals
massive damage, which allows players to fend off opposition harassment or even to opportunistically harass enemy backliners. The choice of offhand is open to preference and circumstance. Dagger offers condition removal, which is far from abundant for Necromancers, along with a decent AE Weakness. Focus offers more damage along with some single target boon stripping and Regeneration. Warhorn offers a Daze and Swiftness.
As for utilities, you're basically looking at taking three from a pool of five candidates. All four Wells are valuable, plus Spectral Wall. Ideally, you'll want some variety among the Necromancers in the zerg; the one utility you want everyone running is Well of Suffering for the massive damage. Beyond that, Well of Corruption is decent damage plus boon stripping, Well of Darkness is considerable control (especially coupled with the Chilling Darkness trait), Well of Power is condition removal as well as a personal stunbreak, and Spectral Wall is simultaneously both a great source of Protection for allies and disruptive to the enemy formation. There's really only one choice of a heal; Consume Conditions is a very strong heal, and it serves a grave need for condition removal. There's similarly only one choice of an Elite; Plague gives you both some survivability and control (this is the major motivation for taking Chilling Darkness).
0/4/0/4/6
You've got the above basics covered, and you've invested heavily in Death Shroud, which serves both offensive and defensive ends. Notably, you've got a 3s Stability on a 6s cooldown, which is borderline overpowered. The Ferocity in the Soul Reaping tree is well utilised as well given the remarkably high crit rate of the build. I've slotted offhand Focus, because Spinal Shivers is such a strong skill for finishing off a single target. For the flexible utilities, I've loaded Well of Corruption and Spectral Wall. In general, Well of Corruption should be the go-to second Well, and it wouldn't be a bad thing for every single Necromancer to have it slotted. Spectral Wall is individually the strongest choice of third utility; as mentioned, diversity is important, and having several redundant Spectral Walls is certainly less valuable than having some Wells of Darkness/Power in the mix, but when in doubt, I'd default to Spectral Wall. We've slotted Sigils of Force and Rage on the Dagger/Focus. Force is straightforward--it's the highest damage option--but Rage is a less obvious choice; Sigils of Rage are a very strong choice for burst damage on weapons with strong auto attack chains, though, and that fits the bill here perfectly. The Staff is well adorned with Sigils of Force and Bloodlust; on-swap Sigils would be a poor choice, as you'll be sitting in Staff for long periods.
This
ALTERNATIVE offers more raw defense from the toughness in the Death Magic line along with reduced Staff cooldowns and a nice boost to Power. The 20-point Major in Death Magic could be any of a few things; Ritual of Protection would be my choice given the importance of the Protection boon, but Shrouded Removal and Greater Marks are both options as well (note, however, that the increased area of Greater Marks can at times actually be a disadvantage in that it makes it more likely that your damage will be spread around among more enemy targets rather than be concentrated).
Mesmer
Mesmers are an unusual class in large scale WvW. They're simultaneously both very poorly suited to this area of gameplay and also completely indispensable. What I mean by this is that Mesmers bring a handful of cooldowns that can completely change the course of a fight if properly employed, but they have no obvious function to fulfil in between using those cooldowns. Unfortunately, they're also rather poor at tagging kills. The upside, however, is that a Mesmer has a great deal of freedom in terms of what gear to wear and what traits to take; all that really matters is what you do with your utility slots and that you stay alive to use them.
So what are these game changing utilities? Well, the one that more or less every single Mesmer should be running is Null Field; a decent group will be keeping quite a lot of quite powerful boons up quite a lot of the time, and stripping those boons is huge for allowing your players to get at the enemy. When you add the condition removal on top of that, you get an absolute dream skill, and you can add even more goodies to it with the right traits. After Null Field, you should be running at least one, and ideally two, of the following three: Feedback, Veil, and Portal. Feedback isn't as gamebreaking as Null Field, but it is still a strong skill, especially when traited, and as an added bonus, it also can help you get tags in. Veil and Portal are both niche skills; having them available is massive, but unlike the former two, adding more and more of them doesn't continue to add the same value. Having 2-3 Veils and 1 Portal is a good target. Moreso than for any other profession, having diversity among what your Mesmers are running is very important, so I strongly encourage you to coordinate with your commander and with the other Mesmers in the zerg. Coordinating your cooldowns with the commander--many commanders will even want to be calling for them specifically themselves--in Teamspeak is incredibly valuable.
So we've got four killer utilities; what should we do for the rest of the build? Well, like I said, you can pretty much play whatever you're comfortable with otherwise. I'm actually going to give you a Condition build here, which is something generally to be avoided in large scale WvW, but it is both reasonably survivable and somewhat less bad at tagging kills.
4/0/0/4/6
We've taken Staff both for Chaos Storm for tagging (and the auto attack at least hits two targets instead of one!) and for the survivability skills. Sword/Focus is the clear choice for your weapon swap with a lot of useful skills on both weapons; Sword is the only Mesmer weapon that cleaves, too. Look at Blurred Frenzy more as a survival skill than a damage skill; the damage is just a nice bonus. Swapping with the clone is also a good escape skill. Traited for reflects, Temporal Curtain and the Phantasmal Warden both help you get tags, and though projectiles aren't the biggest source of damage, negating any damage is always welcome. The pull on Temporal Curtain has all sorts of great applications. Pulling enemy frontliners out of formation (especially after you've stripped their Stability with Null Field) to help burn them down is great, but you can also yank players off of walls and the like.
We've got Mantra of Recovery to work with the Runes of Altruism; with the Mantra heal, you'll be able to trigger the Might and Fury immediately off cooldown. I've slotted Timewarp for the Elite here; it's a reasonably strong support skill, especially when holding a choke against an enemy push. If you're running Portal, though, you're probably going to want to have Mass Invisibility slotted to help you utilise Portal.
We've got Carrion/Rabid gear slotted for condition damage to augment the Confusion you're throwing all over the place along with the conditions from your Staff. This mix provides pretty reasonable defensive stats, but feel free to swap in some Dire if you feel the need for even more durability. The Sigil of Energy choices on Sword/Focus complements the defensive uses of the Sword, whereas the Sigils on the Staff are intended to help a bit more with the damage/tagging.
The main alternative is to basically run a standard Power Greatsword build, in which case you're sort of hybridising your role between backliner and midline harasser, because in between utilising your support skills, you're going to be pressuring enemy backliners with your Greatsword. It's a bit more demanding to play, and you're not going to be getting as many bags, but it's ultimately going to let you have a bit more of an impact and probably be a bit more fun for a lot of players.
If you've got enough Mesmers to get away with sacrificing one of your utility slots for Mantra of Pain, you can also take Restorative Mantras for a 2695 group heal every every roughly five seconds.
Engineer
Engineer Build Framework
Engineers don't really fit all that well into large-scale WvW. You generally won't really find them on any GvG rosters or anything, and it can at times feel a little like the best you can do when building an Engineer is just hope you don't break anything as you jam the square peg into a round hole. If you want to be really super serious about WvW, Engineer may not be the best profession to bring, but if you really can't see yourself playing anything else, don't fear. They may not be ideal, but you can certainly contribute as an Engineer; better than a lot of people give them credit for, even. Do be prepared to completely change the build you're accustomed to from PvE, sPvP, or roaming, though, as most of those skills transfer rather poorly. The Flamethrower a lot of folks are rather attached to is a great way to kill yourself on Retaliation, for instance.
The best way to fit an Engineer into large-scale WvW is to set up more or less a backline support build based around Elixirs. You're going to be running three to four Elixirs with full trait support. You've got thirty points left to spend after decking out your Elixirs, and you've got several options for how to spend them. Because this is a Power build, Rifle is the natural weapon of choice, offering much better damage than Pistol. The auto attacks do pierce, so with proper positioning, they can provide reasonable damage, and the combination of Net Shot and Blunderbuss can be effective for harassing enemy backliners or fending off midliners who try to harass you. Might stacking is a huge part of this build, and your personal Might stacks should be sitting near 25 permanently, so Runes of Strength are a no-brainer. I've taken Sigils of Battle and Bloodlust, but you could take Energy if you need the survivability. On-swap Sigils are all strong choices for Engineers provided you're running at least one Kit, as equipping a Kit does count as a weapon swap.
2/0/0/6/6
With a Sigil of Battle plus Runes of Strength, and taking the Elixir Gun for the final Utility, you should be able to float consistently between twenty and twenty five stacks of Might
without taking Enhance Performance. You could replace the Elixir Gun with another Elixir if you need it, but none of the other Elixirs have short cooldowns on the thrown Tool Belt skills, so they don't offer the same support that the Elixir Gun would. Grenades may also be utilised situationally, primarily when you need to pressure the walls during a siege, but you shouldn't come to rely on them as a staple weapon. If you opt not to run a kit, you should replace the Sigil of Battle with a Sigil of Force. None of your Elites are particularly well suited here; Supply Crate at least offers damage, a Stun, and a Blast Finisher, even if the actual contents don't have much impact; Humans may do well to consider Avatar of Melandru in its stead, which offers a huge survivability boost along with some decent party support and a version of Entangle (minus the damage portion). For the remaining trait points, we've gone deep in Tools, which serves a variety of functions in this build. For one, you're getting decent damage mods out of it--300 Ferocity, 10% crit chance at range, and 10% damage with full Endurance, as well as some more burst damage from your thrown Elixirs--plus a ton of endurance regeneration, which both helps you keep that 10% damage active, and helps keep you alive. Crucially, though, you're also getting 30% cooldown reduction on Toolbelt skills, including the thrown Elixirs this build is designed around.
The core of what you'll be bringing to the table here is condition removal and boon stripping, along with some decent boon sharing. Tossing your three core Elixirs will be stripping three enemy boons, clearing four conditions, sharing three stacks of Might, sharing 5.25 seconds of Stability, sharing three random further boons, and dealing a decent bit of damage, all with only a twenty-three second cooldown. You've got plenty of personal condition removal, massive Might stacking, and modest ranged damage. Not so bad for the black sheep of WvW, eh?
Your further alternatives for the traits here are to invest in the Firearms tree; you can trait Rifle range, Rifle cooldowns, and bonus damage based on the number of conditions on your target here.