Mega Man 5 Part 9: Proto Man Stage 1 + Dark Man I Boss
3Now that Mega Man has beaten all of Proto Man's Robot Master allies, Mega Man can now confront his older brother, who somehow has his own castle.
Also, since I've gotten all of the letters in "MEGAMANV" Dr. Cossack was able to build Mega Man a little bird buddy called Beat.
When Beat is selected, Mega Man can't charge his Mega Buster, but he can summon Beat to follow him around and attack enemies. Beat's seems to hit somewhat hard, and his energy's only drained when he does hit a target.
The boss of this stage is a different Robot Master known as Dark Man I. He only moves around like a tank and shoot at you, and his shots are low enough to still hit you while you're sliding.
To damage him a lot, you could either use charged Mega Busters at him, or just rust him out with the Water Wave.
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DISAPPEARING PLATFORMS ALERT!
There are only two sections that contain these. They're not that hard, but not that easy, either.