Found a fairly easy way to cheese the last fight, as long as you have a teleport spell or two. Did it at lvl 6 with one level 5.
Get your weakest party member (support/healer if you have one) to talk to the Bishop, they won't be attacked once in dialogue. Once in dialogue the rest of the enemies will not attack you until you engage them, so there's no need to sneak and you can set the fight up however you want. (http://postimg.org/image/j2allsu55/)
Keep two people at the base of the stairs, preferably two people with teleport. The worm should pop up like 2 turns in and do his poison spray, just light it on fire and it will create a necrofire. Teleport enemies to the other side of the fire and they will die within a few turns of trying to reach you. If you stand right on the edge of the fire they will stay in it and attack you with physical attacks. The worm seems to prioritize attacking the magisters, it certainly never melee attacked my party, but just in case stand outside of melee range from it. All you have to worry about is the mage/archer. If you can, try and netherswap them with a melee into the fire otherwise you'll just have to kill them normally. Once all the magisters are dead just kill the worm, which should be fairly low since the magisters attack it fairly frequently. Then just heal your party up, end the dialouge with Alexander and kill him.
This may get nerfed but necrofire was doing almost 50 damage a tick, the rogue died in 2 turns trying to cross the necrofire.
