The Summoning Pits First challenge is surviving the the room with the door you have to open. Fall back through the door you came in through after activating the event and work you way through the waves. Watch out for the wizards as they follow you. Fall back to the top of the stairs and pick them off slowly. Their damage is extreme when they come as a pack so make sure someone is always safe. The boss fight is best done from the the previous two rooms. The damage was too extreme at our gear level to survive out in the open. Free the boss and dps him from inside the doorway using the doorframe to LOS his death beam until the shrieker spawns and then fall back to the previous room until it despawns. Rinse repeat killing any foot traffic that ventures in. Dont bother with wizards as they respawn anyway. Leave one person safe in the previous room at all times making sure they dont go too far because it resets the event. You can dps from the back there with a long range weapon if the boss is positioned correctly. This is time consuming method and requires many deaths but it works.

Nexus Pretty straightforward. Minotaurs around the boss are a pain but drop down carefully and immediately behind you is a gap in the wall that you can all fit in. Pull them over one at a time and kill them. Leave 2 guardians in there and one on the ledge to the right as you look out. There is a nice little hollow that means you can attack without dying too much. The two in the gap can also dps the boss but can also thin out the adds. Guardian on the ledge stays on the boss. Use the right elemental sniper and he goes down in no time.