Wily in megaman 7.
7 is easily the most forgiving of the main series games. In addition to the main 8 weapons you get:
Rush Coil, rush jet, rush search, the super adapter which gives you a homing charged shot and lets you fly. You hold 4 E tanks, 4 W tanks and 1 super tank that refills everything. You can even earn protoman's shield which protects you from certain projectile attacks. You can get beat to catch you if you fall in a pit up to 4 times per level.
Most bosses are fairly easy when you know their weakness and only Bass will give you any major trouble. Then you get to wily's second form, who teleports around the screen, often out of reach of your weapons, and each time he appears he launches 4 fast moving, homing projecties at you. Two of them just take off a chunk of health. One freezes you in place. Another burns you for damage over time. They also criss cross each other's path as they move making them very hard to avoid. Wily has no pattern and will still fire these things at you even if you can't fire back.
His one weakness? The coil spring. A weapon that bounces along the ground in an arc. Which you must use to fight a guy who spends 90% of the fight out of your shooting range. That means you need to be in a specific place at a specific time relative to wily to even launch the attack, let alone hit. Most strategies for this fight involve intentionally getting hit by wily in order to avoid his weapons, that's how bad the fight is.
The worst part is that Keiji inafune did this as a prank. He just said to the designers "Let's make the last fight insanely hard" and those madmen let him. They just let him.
Most boss fights, even the hard ones, require you to call upon everything you have learned before in the game. Or at least to call upon the one think you haven't used yet. This one requires you to invent entirely new strategies on the fly AND be prepared ahead of time.
