Plague

Kill three enemies in one fight with Poison (the poison tick has to land the killing blow on each). Silent, obviously - grab Noxious Fumes so every enemy is stacking Poison, then just let the ticks finish them off rather than attacking them down yourself. Any three-enemy encounter works; the Act 1 packs of three (Louses, Cultists) are perfect. Comes on the way to a Poison win.