All the waves and waves of enemies. We COULD wait for Xyor to show up but by then there are tons of Wizards and Knights running around killing us with tons of suppressing fire. Not to mention Phogoth himself...

Me and my team stayed in the entrance way, the room behind where the Shrieker spawns in. The enemies won't go there and Xyor will come right to you. Just chill there, pick off enemies when you can, and make sure you're using the correct elemental weapons for the different shields you'll be up against.

How does the Shrieker work? Does he spawn once and only when you enter there or does he just spawn at a set time?

Well there's two rooms that are important. The Shrieker room, the one leading into Phogath's "lair."(don't know what else to call it) The safe room, the one which leads into the Shrieker room. If you stay in the Shrieker room too long, a Shrieker will spawn behind you. You can avoid him by running back to and staying in the safe room. After some amount of time in the safe room, the Shrieker will despawn and you can poke out to shoot Phogath or mobs as you see fit. After a while, Xyor will show up and drop out of a Tomb ship. You won't get any notification when he shows up, so keep an eye on the wizards' names.