There are two rooms right before phogoth. The first one will spawn a shrieker every 5-10 seconds, stay in that room and shoot phogoths minions that walk to the door way until the shrieker spawns. Once the shrieker spawns run into the room before that and hide behind the wall, rinse and repeat. The shrieker also spawns monsters when it appears and you can also try to kill the minions that flood the shrieker room while its up but you only have ~5 seconds/8 shrieker bursts before they disappear. I got it first try @ level 25 solo doing this, also Xyor (wizard) will appear with ~5-10 enemies left with phogoth alive. PS. You don't need to kill phogoth especially since you get 0 drops. Your weapon upgrades as soon as Xyor dies.