Baal, Lord of the Cheese?

[WarBlade]

Lister's easy. The activation threshold for the monster packs is a couple of steps in from the entryway and the awareness threshold for the monsters themselves is even further inside (you can actually hug that back wall without spawning new monster packs). As soon as you slay Ventar just head back to a point a couple of steps into the room and wait. Lister won't move until you go further in and retrieve him.