The Broken Legion encounter in Destiny's Prison of Elders helps to give the Cabal some much-needed endgame content. PlayStation and Xbox owners will have to defeat Valus Trau'ug, Cabal Juggernaut to get any of the Level 28 through Level 34 loot though.
Note that this guide is only for the boss encounter against Valus Trau'ug. Please check out the Prison of Elders General Strategy Guide for tips on how to get through the first four rounds of any encounter.
Valus Trau'ug's unique feature is the ability to rotate the element type of his shield every time it regenerates. The shield will stay down for approximately 10 seconds once it is brought down, which is when damage should be dealt. When it regenerates, a new wave of minions will spawn.
Unlike the overwhelming majority of the boss fights in Prison of Elders, Valus Trau'ug will come to you instead of sitting in the middle of the map.
This boss fight is about dishing out as much damage as you can in a short timeframe, so choose your Guardian sub-classes appropriately. The Titan Defender and Warlock Sunsinger sub-classes are favored due to their ability to provide protection buffs and resurrect respectively.
The Hunter can go either way though as the Golden Gun super is good for clearing out the minions. The Bladedancer's invisibility was more useful in getting to the fifth round than during the boss fight. It can help revive players in a pinch.
All three Guardians ideally should have a different element type weapons as their primary weapon and secondary weapon because Valus changes his shield element type.
There are no real opportunities to snipe in this battle so shotguns should be favored.
Heavy weapons can be saved to put as much hurt on Valus during the time his shield is down or for enemy clearing in a pinch.
Titans will want the Helm of Saint-14 if possible. The blinding effect it gives the Ward of Dawn bubble is extremely valuable to help control minions that rush.
All three Guardians should head to the room on the left side of the map and focus on clearing out minions before attacking Valus Trau'ug and his shields. The last thing you want to do as a group is to bring his shields down multiple times only to find yourself overwhelmed by the number of enemies on the map.
While behind the wall in this room, you only have to worry about splash damage from Valus' cannon and the occasional minion rusher. Valus and the vast majority of the minions will stay outside the room.
The Titan(s) on the squad should place their Ward of Dawn bubbles in middle of the room or on the two openings on either side. These are the areas where Cabal Centurions and Legionaries are more likely to rush.
If you do get overrun on the left side, you should rotate to the opposite side of the map until the number of enemies is reduced to a manageable level. They can then rotate back to the room on the left side.
Once the minions have been cleared, everyone should focus fire on Valus; use the element type he is currently shielded with if possible. Once his shields are down, switch to the weapon that dishes the most damage output, typically your best rocket launcher or shotgun if you are daring. When Valus' shields come back up, all three Guardians should stop shooting at him and should again turn their attention towards clearing out the new wave of minions.
Communication on which shield type Valus is using is important to make sure everyone is on the same page. It's also key to call out when his shields are back up so that they aren't accidentally brought down a second time, which spawns another round of minions.
Again, one of the biggest mistakes players have seen when fighting Valus is players trying to bum rush the boss and take him out quickly. They don't realize that his shields going down and coming back up spawns another wave of minions. This boss fight isn't particularly hard as long as you keep it to one wave of minions at a time.
All Gally Strategy
If a team is equipped with all Gjallarhorns and is high level, it is entirely possible to bring Valus down quickly. The easy way to do this is to clear out the first wave of minions then unleash shot after shot from every team member's Gjallarhorn until he is dead and ignore the regeneration of his shield.
If Valus' shield is not Solar, it's helpful to bring it down with a primary or secondary weapon then unload into him with the Gjallarhorn so as not to waste any shots.
You can kill Valus either before or while the next wave of minions spawn. If you are unable to, simply clean out the worst of the minions and then unload the last of the Gjallarhorns into him. He should be pretty close to dead already.