Perfect

Kill any boss without losing a single point of HP. Note HP lost to your own cards (Bloodletting, Offering, Hemokinesis etc.) doesn't count against you - it's specifically damage taken from enemies. So either block every incoming hit for the whole fight, or burst the boss dead before it acts (see You Are Nothing). The Act 1 bosses are the easiest targets - low HP, simple patterns. A big-block or high-burst deck gets there.