Champion Enemy!

From wave 25 on, each non-boss wave has a 15% chance of bringing one champion. It has 2.5x health, costs double lives if it leaks, and comes in one of five kinds: regenerator, splitter, shielded, haste aura and necromancer.

Kill it for +20 gold, 10 units of a random material straight into your stock, and a guaranteed rare-or-better card next wave.