Shellbreaker

Shellbreaker is a pink orb. He will repeatedly fire boomerangs and rainbow rings while flying around the stage. His boss room is 2x2, which is the same as a later boss, Space Box. At 1/2 health, he will turn purple and will fire boomerangs faster, as well as larger clusters of rainbow rings. At 1/4 health, he will turn red and will fire boomerangs even faster, as well as even larger clusters of rainbow rings. After you defeat him, you will obtain the boomerang.