The AI seems to really hate walking through fire/over grease. Lets say you wanted to take out all the gobbos partying around Volo. Just aggro them, back towards the bridge with the palisades and put down grease on the bridge side of the gate and wait. More or less all enemies clump up in the doorway and just wait to be AoEd to death. Same strat can be used to kill Dror Ragzlin and his guys (just put grease/fire in the dorrways and they won't leave the room) or the goblins at the entrance to the goblin village (just back away over the bridge)
Sure not ALL will stay put but moste of them will making it trivial to kill the few hard-hitters with HP enough to dare cross a AoE fire or a patch of grease.