Wait unless im reading this wrong you never got to the 1v1 fight against Senator Armstrong?  Dude if thats the case power through the mecha spider cause that fights is amazing!

General RAAM from the original Gears of War. That enormous wall of difficulty left a terribly very sour taste in my mouth. It's a good thing the sequels were such vast improvements.

There is one boss that I've never been able to beat that I put a lot of effort into. Hecaton from FFXIII. Seriously the worst. I fought him about 10 times without looking up a guide. Then I watched upwards of 10 or 20 guides, and followed all of them to the letter. There were two very distinct differences between my fight, and every other one that I saw. One: I was over leveled like crazy. There seriously wasn't one video where the player was leveled higher than me. And I was probably 30-50% higher than the average person. Then there was the second difference that I noticed. Every time I fought hecaton, he did an attack that would always drain my health to zero. This is because this one attack was essentially all of his attacks uninterrupted. If he didn't perform this attack, I would have won on my first try. Eventually it got so annoying that I never played the game again. And after watching the ending, I don't regret my choice.

Ryu Hayabusa in Ninja Gaiden 3. His cheese made me sell the game. Can't think of another game I haven't bothered to finish because I got stuck.

The final boss in the original Skylanders game. He was just to damned hard. Seriously, who thought having such a hard end boss in a game meant for families was a good idea.

Last Remnant was probably the game where I had the hardest time with bosses. I was able to beat all of the main bosses, but i could never beat the side quest boss "the fallen."

Recently, the boss below from Bravely Default comes to mind.

I plan to go back to it, but after being defeated effortlessly 5 times in a row after beating the game and Chapter 8's boss gauntlet, I didn't feel all that motivated. Someday....*glares at the evil fox*

I def. beat it on normal difficulty but possibley Hard!  I'd have to go and check to be sure, heh.  But yeah the race one is insane as are most of the last ones

I can't beat the 3rd form of Lavos in the final battle of Chrono Trigger DS. Also the boss Fiore the Rosa and a giant water boss whose name I don't remember from Bravely Default. I'm really bad at JRPGs in general.

The boss from Kingdom Hearts Re:Chain of Memories(part of 1.5), the one that used Ice, Vexan. My god I seriously haven't raged like that for a long time ,most of those bosses were really hard but Vexan really pissed me off.