Source: Dulfy (and thanks to all the Oops'lings who helped create the class-guides and changes to the boss-guide).
VALE GUARDIAN GUIDE
You need to have a ranged party of 4 players whose job is run to the green circles as soon as they spawn. Druid, and ranged classes are great for this.
The tank is typically Chronomancer, other classes should have 1K toughness only.
Additionally, you need to have a party of 2-3 condition damage dealers to go to the Red Guardian when the split phase happens at 66% and 33%.
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Red Seeker Orbs spawn every 20s
Teleport blue circles spawn every 9s and persist for 2s
Green circles spawn every 15s and takes 5s to charge up.
AoE sector change every 20s
Breakbar AoE occur every 30s
The fight is divided into 5 phases. Two of the phases are split phases which occur at 66% and 33%. Vale Guardian has 22 million HP and an enrage timer of 8 minutes.
This phase will last until the boss reach 66%.
The Vale Guardian fight area is a circular area divided into 3 equal sectors, each with a different colored pylon in the middle (Red, Green and Blue).
At start of the fight, the tank should drag the boss to a pylon next to the Speed Mushrooms and keep him stationary to ensure max DPS. Do not attack the boss until the tank has him positioned properly, as initiating combat will start the timer. Tank can avoid the teleporting blue circle circles by dodging 1 second after they appear back into the pylon wall. Remember that "Speed Mushrooms" buff also increase your attack speed by 10%.
The boss does have a cleave melee attack so you will want to face him away from the raid. However, he does turn when he spawns the blue teleporting circles but he will not cleave the raid so there is nothing for the melee to fear. The arrow undernearth him will show you which area he is facing.
Red Seeker Orbs
Three red seeker orbs spawn every 20s directly next to each of the color pylons. Their goal is to move towards the players and deal significant damage to them. They are immune to blind but you can use immobilize or knockback to push them away. This is especially important when they migrate towards players standing in the green circles.
Blue Teleporting Circles
These blue circles spawn every 9 seconds usually within melee range of the Vale Guardian. They last for about 2 seconds and then they will explode, dealing damage and teleporting anyone caught in them to a random location. Dodges/blocks/evades all work against them, and usually you have enough time to simply sidestep away from them. They can become problematic if they spawn within green circles and teleport players out of it.
Green circles spawn every 15s in a random location in the same sector as the boss. You have about 5 seconds to get 4 players into them to negate a raidwide damage. Each player standing inside the circle will reduce 25% of the raidwide damage. There is an inner green circle with a lightning orb on top that get smaller and smaller and before it become a lightning flash. If you don’t have 4 players standing in the the green circle when this lightning flash happens, your raid will take damage. Downed players do not count so if you have 4 players and one of them gets downed while standing inside the circle, your raid will still take the damage. Even with 3 players standing inside the circle, the raid wide damage is enough to down anyone not in full HP, and can cause an unrecoverable mess.
This green circle is usually the number one reason for raid wipes. While simple mechanically, the green circle could spawn in a far away location, or players standing in them could get teleported by the blue circles or get downed by red seeker orbs. Therefore it is very important to use your knockbacks if you see the seeker robs approaching to keep them away from the green circle. You need to assign a ranged team of 4 players whose job is solely to run into the green circles as soon as they see it spawn and someone on voice comms will need to call them out when they spawn.
Additionally you can have 2 mesmers blocking the dmg by timing their skills before green-circle-dmg hit, and you can all ignore green circles. This is only advised for an experienced team and for mesmers who knows those rotations.
PHASE 2 (split)
Phase 2 triggers once the boss is at 66% HP. The Vale Guardian will become immune to damage and run to the middle before disappearing. You will see lightning beams connecting him to the three pylons during this phase transition.
You should have decided pre-fight which members of your raid team are going to each guardian. For the red guardian you will need 2-3 condition damage dealers. Only 1 need to go to the green guardian while the rest of the raid can stay at the blue guardian. You only have a few seconds to get into the right colored section to get the attunement buff. Once you get this attunement buff, you will receive massive damage if you are inside the red circle of a different colored guardian unless your guardian has being killed. Therefore, you must stay by your assigned color guardian at all times unless otherwise instructed. Many raids prefer to bring Green Guardian straight to Blue Guardian.
For Blue Guardian there are two key mechanics: boon strip and green circle.The green circle mechanic is identical as the one in the first phase but you only need 3 people inside the green circle. The green circle in this phase will spawn pretty close to the Blue Guardian so you should have no problem getting into it in time. For boon strip, the Blue Guardian gains a buff that make him immune to damage every 10 seconds. You will need to remove this via boon strip. Chronomancer sword autoattack will do this job but Revenants can also boon strip in Mallyx, or use the sigil: Sigil of Nullification
Once the Blue Guardian is at 0 health, he will gain a breakbar that you will need to remove in order to finish him off. Do not waste your big crowd control abilities here as you will need them later in Phase 3. If you run two Revenants, one of them can use their Sword 5/Axe 5 and then Staff 5 to fully break it.
Initially Green Guardian will be tanked at the green pylon. Once the Blue Guardian has being defeated/or close to die/or just move right away (depending on the team), the tank should drag the Green Guardian to the Blue Guardian. The Blue Guardian team will take a bit of damage from the blue orbs since they are standing inside Green Guardian’s AoE zone but the damage is healable. The Green Guardian’s special attack is the blue teleporting circles so make sure you keep an eye out for them. Like the Blue Guardian, there will be a breakbar to break at end of the fight.
Red Guardian is condition team’s job (2-3 players). The Red Guardian can only be damaged by condition damage so power DPS are useless here. Watch out for the Seeker Orbs. Break the breakbar in the end as on the other Guardians. If you somehow finish early, prepare for the Vale Guardian but do not help Green Guardian team unless they need it.
Phase 3 lasts from 66% to 33%. It will have the same mechanics as Phase 1 with two new mechanic additions. Due to the movement in this phase, warriors are advised to drop their banners in the middle of the circular room.
Starting with the Green Sector and rotating clockwise, a sector of the room will be covered in AoE for 20s at a time. The rotation can be Green –> Blue –> Red. Some tanks prefer other rotations, so this is not the only way to do it.
While this sector is covered in AoE, anyone traversing through it will take significant damage and can be downed if they are not at full health. Need less to say if you get teleported by a blue teleporting circle, you will have a bad time as there is a chance that you will get ported to an AoE covered sector.
Tank should start the boss at the Red Sector and then drag him clockwise to the next sector as soon as possible. You basically want to kite the boss to the intersection with the next sector and drag him into the next sector with 3 seconds before the sector AoE change. The number 1 reason for wipes in Phase 3 and 5 is because the tank does not drag the boss fast enough to the next sector. If this happens, the green circle can spawn in the previous sector where the AoE occupies and your raid team can get downed trying to get to it, resulting in a raid wipe.
If you get a bad green circle, ask your team to not going there, and instead stack together. Use rebound and other skills to block the dmg, and heal through it.
This is the second new mechanic for Phase 3. Every 30s, the boss will stop and gain a breakbar. While this breakbar is active, the boss will constantly spam AoE orbs at the raid. The melee DPS must save their big crowd control cooldowns for this and destroy this breakbar as soon as it appears (basically within 1-2s). You cannot count on the team there are going for green circles and they may not be in a position to help you break this bar.
PHASE 4 (split)
Phase 4 is identical to Phase 2 split. The big thing you have to watch out for is that a green circle may spawn right before this phase transition happens, forcing you to stand in it while the transition occurs. This can really screw over your team split. Your team may have to make an on the spot call on who will stay in the circle and who needs to go to their sector to get attuned. You absolutely need 2 condition damagers in the red sector and a boon stripper in the blue sector. Swiftness and superspeed can help you to make a last minute dash to the right sector.
Lulle Art from what happens in last phase
Phase 5 is where most of the wipes happen as it is the most difficult. A single mistake can make the difference between a successful kill and a wipe. There are no new mechanics but several of the mechanics from Phase 1 and Phase 3 are modified to be more difficult.
There is an interesting strat for phase 5 which would remove lots of the RNG and make the last phase a bit easier. Basically, you do not stop the Vale Guardian’s Raidwide AoE which requires a breakbar (do not use any crowd control abilities at all). While the Vale Guardian is doing his AoE attack, he won’t do his green circle, red seeker orbs, or blue teleporting circles. Basically you position the boss in the middle and just attack him while switching between the sector AoEs. You do have to watch out for the seekers orbs that spawn before he enter the breakbar phase as they will try to run towards the raid and deal massive damage.
Soft CC like slow/cripple/chill etc does eventually eat away breakbar and the green circle will spawn immediately after you breakbar him.
Instead of having a single Sector covered in AoE, two sectors are covered in AoE. At start of the Phase 5 it will be Red sector that is safe while Green and Blue are covered in AoE. Tank will need to do the same thing they did in Phase 3 and drag the boss into the next sector about 3 seconds before the AoE sector change.
Unlike Phase 1 and Phase 3, 4 Seeker Orbs spawn in this phase, with the 4th one spawning in the middle. This makes seeker orb management much more important so use those knockbacks and immobilize to control them away from the green circles and the melee stack.
Enrage and Downed State DPS
Enrage timer is at 8 minutes. After 8 minutes the Vale Guardian will do 200% more damage. It is still possible to kill the Vale Guardian even if you are past the enrage timer. Downed State DPS is also crucial so if the boss is low and everyone is downed, do not give up. There are tales of a single warrior getting up from Downed State with Vengeance and finished off the boss.
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