I usually take him out by teleporting my party up on the left wall (the side all the seekers run to once you kill the shriekers) as long as the bishop doesn't see you it doesn't trigger. I position my party in a way that makes the enemies have to funnel themselves to me. They all moved first but I was able to attack first so I slaughtered them at level 6 then when the worm comes out depending on how my party is doing I either fall back and use the worm as a distraction or I just finish everyone off. I easily defeated the bishop and worm in two playthroughs the first fight was rough the second one was easy but I had a stronger party.
