[Aerich]

Spoilers
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For me, the most efficient tactic on this level is to start the fight at the entrance (as Marty and Morgeruat suggested). Drop Web, Entangle, Stinking Cloud, Spike Growth, Skull Trap, etc. in the hallway before you expose Albion, then take him and the librarian out ASAP. Next, hammer on the enemies that are (hopefully) caught in your kill zone. Considering the trolls, Fireball is a good spell here. Don't be shy about using potions, either, especially fire potions.

After that, if you don't have to rest, charge off to the right and take out the high summoner. If playing on higher difficulties, watch out for multiple yuan-ti mages.

The rest of the level should be easy.

I have never let the adventurers out before clearing the level, so I don't know how effective my preferred strategy is compared to freeing the allies.