Darkwing Duck/Floating Fortress

Boss: Steelbeak
Steelbeak is both the hardest boss in the game and the only one with two parts (essentially twice the health).

Part One
When you enter the room, Steelbeak will be sitting in a control room in the upper right. Two small flying mechanical units will attempt to destroy you by running into you and firing small purple projectiles at you. Your goal is to destroy the window noted by the two glowing markers on the left side; it can be destroyed by shooting between them. There are two ways to do this: you can either use Thunder Gas and fire from the ground near the bottom left corner of the room, or you can (and probably will have to) climb up the candelabras and fire normally at it.

There is just one simple way to get through this without dying: first kill the two robots that start in the room. Try to kill them one right after the other. Immediately climb the objects on the wall and hang onto the one on the upper left. Face Steelbeak, and just keep firing. Whenever a robot respawns, destroy it while it is flying downwards on its starting path (during this small amount of time, they won't try and attack you). This way, you'll be able to kill off the minions while you go after Steelbeak.

Eventually the glass will shatter, a full health box will appear, and Steelbeak will take an elevator down to the floor to face you himself. Remember to heal if need be.

Part Two
Steelbeak will begin by running back and forth like a crazed chicken. If you drop to the floor to get a shot off at him, be ready to jump! After getting hit he'll pull out a large metal block and throw it at you before pulling another one out and using it as a shield. While he's using it as a shield, climb to the top left or right wall object and wait for him to reach the opposite side of the room. Drop down while he's running towards the wall (not after he's reached it), pop off a shot, then jump back on the wall. If done right, he'll throw the metal object away and begin running around again without any defense. Simply repeat this process to win!