Once you get to the summoning pits, you can stay in the entrance cave where the Shrieker spawns. no need to kill him, he will despawn. the knights and wizards will come partway into the tunnel. Save the correct damage type grenades, supers, heavies, and specials and take them down. You won't even need to worry about Phoggy. Eventually, Xyor will come into the cave, and he is just another yellow health wizard.