The titular The Guy from I Wanna Be The Guy.
Phase 1, easy timewaster. Stand in a non-stupid spot and you won't die.
Phase 2, don't fluff an easy jump timing and you won't die.
Phase 3, don't fluff an easy jump - double-jump combo and you won't die.
Phase 4, double jump shoot three times double jump shoot three times etc etc. Can't die at all, easiest phase in any boss in the game.
Phase 5, SUDDENLY CONTACT LENSES. Because of the extra shot and the possibility of missing and the increased fireball frequency, you can actually die here!
Phase 6, CONTACT LENSES, FIREBALLS, TEETH MISSILES AND EVERYTHING IS ON FIRE AND THE BOSS HAS ADAMANIUM EYEBROWS OF INPENETRABILITY. The only part of the fight that is genuinely hard, and it's only made more frustrating because it takes a repetative 5 phases to even get to this point again. I've all but given up because the fight's just so damn boring.