Ooh Donut!

Very specific: land the killing blow on Donu with the Ironclad card Feed. Donu is one half of the Act 3 The Shapes boss pair (Donu and Deca). Feed permanently raises your max HP when it kills an enemy, and this wants that kill to be on Donu. So: play Ironclad, carry a Feed, reach the Donu & Deca fight, whittle Donu down to within Feed's damage, and finish Donu with Feed (not Deca). Leave Deca or kill it after - the important bit is the last hit on Donu being Feed.