The very last Foot in Snake Rattle n' Roll. A friend and I used game genie to finally get there and ended up having to spend about an hour of constant tounge lashing just to kill him. And once you finally kill him, all you get is a "Join us again for SRR part 2" which was never even made.
Dr. Wily's last form in mega man 7 is insanely hard too.
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