Sturm - Advance Wars: It took me three times to get to the end of the game due to difficulty and when I finally did I ended up with the three worst possible characters: Andy, Grit, and Sami. You're already at a severe disadvantage when fighting Sturm, he starts out with way more troops, bases, and money than you and then you have the three worst characters to fight against him. Online guides even tell you that their strategy might not work, so good luck.
smurf this guy.

Ozma - Final Fantasy IX: It only took me three tries on my first playthrough, but every subsequent one, it's taken me a while and I think I've only ever beat him twice because part of it is luck. I think I got lucky on my first playthrough.

Aesir, God of Chaos: This is just a recent one. I played Bayonetta 2 recently and found the game's final boss to be tough.

Clay - Pokemon Blaze Black: Blaze Black is a Pokemon hack that adds more Pokemon to the game's and increases the difficulty and I think Clay was the hardest boss I fought. His pokemon have stronger attacks than you, he starts off with slipstream, his seismitoad busts out coverage moves, and that mothersmurfing Excadrill. That Excadrill! Swords Dance all day. I think I had to go and get me a Swanna and teach it Rain Dance in order to beat him.

Defender and Factory Chief - Super Mario RPG Revolution:

You really think I'm going to spend all this time trying to beat you?
https://youtu.be/aF8quLKkk38?list=PL...1voj-wtE&t=285