Gabriel from Star Ocean 3. He was a secret boss in a dungeon and if you lost you had to go through it all over again just to get back to him.
Sephiroth from Kingdom Hearts 1. I basically just fought Rock Titan over and over again until I was at the right level to beat him.
The Senator from MGSR wasn't that hard, it's just I could never get past the part when he throws the buildings at you and then the slow motion starts and you have to line them up perfectly or you pretty much die. I looked online and saw you could just run past the building and not take any damage. After I found that out, I beat him pretty easily.