Mega Man 2/Wily Fortress 5

Ahh, the infamous "you have to fight all the Robot Masters again" stage.

You'll start this stage in the Dr. Wily Teleport System room. Beat all the Robot Masters again the same way you did before. Shouldn't be too tough. For a few giggles, try throwing Metal Blades at Metal Man.

Once they're all dead, a ninth teleporting hatch will appear. Go through to fight Wily's contraption.

This battle has two parts, which are both relatively simple and similar. During phase 1, if you have any Metal Blades left, unload on the machine with them using jump attacks and diagonal attacks. After the first life meter depletes the shell of the machine cracks off revealing Dr. Wily, and the life meter refills. Continue firing Metal Blades at him using a jump attack. If you happened to defeat Fortress 4 with 1 Crash Bomb remaining, during phase 2, try to get on the back side of the machine, and release the Crash Bomb directly on Wily as he is backing up. If placed correctly, you can defeat phase 2 in one hit. This is very hard to do and will probably require you to use 1 or 2 energy refills.