Hmm. Okay, so the room before the main chamber, that's what we'll call the Shrieker Room- this is where you'll be doing damage to Phogoth from to cheese him. Very little will come into this room and the stuff that does won't come in very far. If you stay toward the back half of it, you can still hit Phogoth through the doorway and don't have to concern yourself with adds except the ones that actually come up to the door. However, every so often a Shrieker spawns (you can foretell its coming by the red dot). When this happens, run back to the room before it, not into the main chamber, the room BEFORE the Shrieker room on the way to the boss. You're safe in there. Don't go back further as it can reset you, but that room is safe, just hug the right wall. The Shrieker comes with Wizards and Knights, none of which you have to kill- they all just disappear along with the Shrieker, so save your ammo. Then when it disappears, go back into laying into Phogoth and repeat. If you stay on top of the Shrieker spawns, it's a 100% success rate, the only variable is time.