I mean...as unexciting as it is, I think the fastest way to "beat" the last fight is to interrupt the timer as little as possible.

At least on this difficulty (I assume it applies everywhere, but I've only tried on this difficulty), the last fight is impossible: After you knock him down twice in one round, he'll become invincible until the next round starts. Feel free to watch this clip, where I knock him down with my special, knock him down with three hits, then hit him three more times with no change to his health:

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