Tunnelbot

Gakin (ガキーン), which you confront at the end of Marble Garden Zone Act 1, is one of the easiest sub-bosses in the game. Just like Sonic 2's Drill Eggman II, this boss burrows through the ceiling and back. Hit it whenever you can. With Knuckles, there is a rotating spiked platform that you can stand on to compensate for Knuckles' lower jump height.