With Doc Robot out of the way, you will now face Proto Man. That's his name in later Mega Man games, but here he is called 'Break Man' instead.
You've been fighting him throughout the game thus far, and the strategy hasn't changed significantly. This fight is actually almost easy, in fact.
Just stand on the central area and shoot him as he walks back and forth. The only time you should stop shooting is when you turn to slide under him as he approaches from either direction. Then it's back to shooting.
Though he doesn't have a life meter, you should find that it won't take long to hit him with enough force that he leaves the scene. You're now ready to begin the assault on Dr. Wily's castle. The password immediately below is the last you'll be able to use, and starts you at the front gates with nine energy tanks.
Password: A-1 (blue), A-3 (blue), A-6 (red), B-2 (blue), B-5 (blue), D-3 (blue), E-1 (red), F-4 (blue)
NEXT: Skull Castle