Urschleim

Touching any part of his body deals twenty thousand damage that ignores armour and the cap on damage per hit, and it kills other monsters in his way too.

It puts these on the ground:

UrschleimWall Boss BulbWall Boss Head

He circles the core endlessly and cannot be hurt, and the only things that take damage are the glowing bulbs riding on his back.

With every bulb popped he stops, and his head surfaces one segment short of whoever killed the last bulb.

His head bursts out of the ground, smashing every tile in an eleven by eleven square, and stays exposed until the timer runs out or you take a quarter of his health off in that one window.

He sinks back down and every bulb grows back at full health.

Left alone he heals a tenth of his health every five seconds until he is whole again, and the more hurt he is the faster he circles.