While you, most likely, will just unlock this by accident, there are two easy ways to force the achievement.
1: Trapdoors. If you can handle the pressure, these are likely to have one to two minibosses. If one of these is a red dragon, you can either coax the second miniboss into getting shot, or kite said dragon to the miniboss of the floor.
2: Possess a red dragon with Chaunter, and pray you can find another miniboss before you run out of time.