Element Lords

If you use only elemental-themed towers from rounds 26-50, round 50 will have this quartet as bosses.

Terra: Terra is the master of Earth. He has always been tough, but never very smart, and his exposure to extreme heat and pressure turned mere ceramic to rock. He has 500 HP. Earth attacks HEAL him one HP, but water attacks erode him, dealing double damage. Air attacks are ineffectual. He can create shockwaves for 7 damage in an AoE. He can also pummel 20 HP out of one tower.

Aurae: The only woman in the group, Aurae is the mistress of Air. The accident that gave the four their power resulted in her being ripped apart by a hurricane inside her body, only to turn into an air elemental. She has 275 HP. She is weak to earth but healed by air. Fire attacks do nothing. Aurae has a 5-10% chance to, after being hurt by a projectile, propell it at a tower for 2 damage. Energy is simply rendered useless instead. Aurae cannot do this with bombs, and bombs render her unable to move or use her deflection for a moment.