This page covers general recommendations for encounters in the Prison of Elders. Included are strategies for different boss rounds.
Room Strategies
Fallen
This room is a large, open area that contains a partially enclosed hallway around the side closest to the entrance. Most enemies come from doorways along this hall. The left side is raised higher than most of the arena and has only two ramps, making it somewhat defensible. The right side has a single, smaller raised platform with no ramps, meaning no enemies can walk up. However, cover is somewhat limited. In the middle are several pieces of debris that can be used as platforms, or as tunnels underneath. The back edge furthest away from the door is very dangerous to traverse, as it has no cover and lots of exposure. Do not attempt to use this pathway unless invisible or enemy presence is very light. Any bosses in this room will be able to relatively easily wander around the arena.
Splinter mines will appear on the left and right platforms, and one of the debris platforms in the middle. Finding a path that is relatively light on enemies will help teams to reach the mines and disarm them.
Hive
This room is smaller and enclosed. The left side is slightly raised and has tall cover available, making it useful in certain boss fights. It also forces all enemies to take one path The right side has three different paths of approach and very low cover, meaning that enemies such as Wizards and Ogres can have an easy line of fire. In the middle is a raised hill and a shallow pool of water; this area is a major killzone as Ogres and Wretched Knights spawn here, and Wizards and Boomer Knights can easily target you here. Unfortunately, it is also a potential splinter mine location. Because Thralls tend to make hallway cover less effective, finding areas to funnel them is a safer option.
To deal with splinter mines, teammates may need to use invisibility or a ward of dawn to deal with the central mine, as its exposure to heavy fire is a major issue. The other two mines appear on the left and right platforms.
Vex
This room is a large, open area that contains a partially enclosed hallway around the side closest to the entrance. Most enemies come from doorways along this hall. There is also a raised walkway above the center of the room with limited cover. There is a large broken vex gate in the middle; most minotaurs and bosses will appear in this area. The left and right sides have some cover, though larger enemies such as Overmind Minotaurs will be able to fire down from the center if Guardians are not crouching.
Splinter mines will appear on various raised blocks throughout the map. Positioning yourself to avoid hobgoblin and minotaur fire is the key to survival.
Cabal
This room is smaller and enclosed. The center has a raised platform that looks out over a larger open area where most Cabal appear. The area immediately between this central platform and the entrance is fairly narrow and can serve to funnel enemies. The left side has some eye-level cover, and can only be approached from two directions, one of those being the narrow, funneling pathway. The right side has a tower which can be walked through, climbed on top of, or walked around. This area can provide short-term protection but is otherwise not easily defensible against a large number of enemies approaching from the center of the room.
Splinter mines can appear on the central platform, as well as the tower area.
Boss Strategies
Gulrot, Unclean
Gulrot's main ability is to spew bile in the air, which slows down players (similar to a web mine explosion). The key to staying alive is finding adequate cover while the bile lasts; the left and right corners of the room offer some protection. The left corner is generally safer as it has higher walls and funnels enemies down a narrower path.
Keep in mind that like Phogoth, Gulrot's weak spot is on his chest, not his head.
Kaliks Reborn
Kaliks fights just like Sepiks Prime, with the added twist of being able to regenerate health. Look for glowing purple Dregs called "Archon Servants." They will mainly appear from one of the doors nearest the arena entrance. Kill as many as you can before Kaliks consumes them to prevent it from regenerating health. Once all Archon Servants have been dispatched, weed out the remaining enemies to give you some breathing room.
Once Kaliks' health is down 50%, players will be required to kill a critical target. Do not be dismayed if Kaliks regenerates; this will only happen once. Continue until Kaliks is destroyed.
Valus Trau'ug
Be sure to have everyone on the team equip one of each weapon damage type before entering this fight. Trau'ug has a shield that will change colors every time it regenerates (which is about 10 seconds after the last shield is disabled). Because of your narrow window, maximize your damage output while you have the chance.
Qodron, Gate Lord
Qodron will mainly stay in the middle; the thing to look out for is a glowing blue Goblin called "Qodron's Eye". Get Qodron's Eye's health down as much as possible, but do not kill it until the message "Qodron searches for targets" appears. Killing Qodron's Eye grants the "Jailbreaker" buff, allowing one player to easily destroy the detainment bubbles that appear every 40 seconds or so. Without it, it is very difficult to destroy the detainment bubbles before they kill you.
So long as the team is staying alive and destroying detainment bubbles, chip down Qodron's health until he is dead.
Urrox, Flame Prince
Urrox will stay close to the pool of water where he appears, and is able to lob boomer projectiles over long distances, so take cover. The best place to take cover is on the left hand side; this place is also good for when Urrox activates his searing flames attack: for ten seconds he makes the ground cause damage, requiring players to jump in the air to avoid damage. The barriers on the left side are the only ones high enough to completely protect players while jumping. A Defender Titan's Ward of Dawn bubble with Blessing of Light greatly helps in this instance. Recommended weapon loadout is ascended Solar-based weaponry (as there is typically a Solar Burn), a primary (e.g., Vision of Confluence), a special like Ice Breaker or Black Hammer, with a heavy weapon like either Hunger of Crota or the Gjallarhorn.
Skolas, Kell of Kells
This fight predominantly has two phases. During the first phase, players cannot do full damage against Skolas until they kill one of two glowing blue servitors called "Kaliks Minor". Skolas himself will wander all over the arena armed with a Scorch Cannon, so be sure to find adequate cover and avoid his blasts.
When his health goes down to 50%, players will no longer be required to kill Kaliks Minor and instead dismantle mines. However, Skolas will also cast a debuff called "Devouring light" which will kill Guardians within 30 seconds. The only way to avoid death is for another person to pick up the debuff; the person who previously had the debuff will be immune for 35 seconds, meaning that the third player needs to take the debuff off the second player before they are killed. If one player with the debuff is killed, Skolas simply casts it on another player after a few seconds. It's crucial to revive the downed player sooner rather than later, due to the need to pass on the debuff. Stay close and on alert to make sure all three players stay alive. Players will be required to dismantle mines a second time once Skolas' health goes down to about 25%.