Wily from Megaman 7 was a real tough final boss for me. I had to use all the E tanks I had to beat him, and the special S one too, and I'd still die. >.

Trainer Red

Probably the boost guardian in metroid prime 2 on the gamecube (because in the prime trilogy it's way easier)

Sephiroth in both Kingdom Hearts 1 and 2. Fuck him.

Yiazmat from FFXII by virtue that it has 50m HP and the fights takes hours. If you get distracted 1 sec you get wiped.

Glitched Alduin (first time you fight him) so that he was invulnerable. That fucker wouldn't taje any damage, regardless of what I did! And it wasn't like the glitch fixed after dieing once, no no no. When I understood that I simply couldn't hurt him, I checkedoine to see if any others had experienced the same thing. Quite a few had. Eventually I had to reload a save 2 hours back and do it over again.

As for non-glitched boss-fight... I dunno.

Albert odyssey on Sega Saturn. One of the bosses literally took an hour to beat. My whole team died but my dragon character was immune to poison that killed everyone else kept reviving team and slowly attacking when I could. This is back when JRPGs requires grinding which I did plenty of.

Roses are red

Violets are blue

I'm a schizophrenic

And so am I

This edgelord:

Rodin from Bayonetta 1 & 2. Seriously, borderline to almost impossible.