This one is actually pretty easy with the sorceress once you reach level 5 and get your first area affect spell. Go back to the first dungeon. Everything should be a one-hit kill for you now and you should be durable enough to take a beating for a while. Find a large room with a large number of creatures (the respawning ants can help fill a room pretty quickly). Run around the room and get everything chasing you. Let them swarm you in the middle of the room, and then use the area effect spell of your choice once or twice. If you had enough creatures chasing you, the achievement should pop for you instantly. This could probably be done the same way with any of the alternate forms.