The Crow

Defeat the Awakened One, the third Act 3 boss (The Time Eater and The Shapes are the others). It has two phases - kill it once and it revives at full HP with bonus Strength - so you need enough sustained damage for two health bars, not one big burst. It also gains Strength every time you play a Power card, so go light on Powers in this fight. Act 3 boss RNG.