Good, but...
I don't understand the concept to fight Mysterious Foe. Are you supposed to just last long enough?
The Twinkle Shot, the only thing I can get to hit him, doesn't reduce his health. He's continuously above firing range otherwise. The present gun seems effective as a spring and one-shotting its orbs, but whenever I jump on it, it blows up.
Is it SUPPOSED to be impossible? The other four bosses were well-designed, and the items were placed so that taking the easy path of weaknesses denies you an E-tank, something I like. Good design overall, but... THAT FINAL BOSS. UGH.