Demon Wall and Elder Wyrm were both tough. But if I can include hunts in this, then Gilgamesh v2 was the hardest fight of the game for me, because I went immediately from the first to the second fight without thinking about if he'd be at a higher level. I got my ass kicked the first time I faced him, but I was too stubborn to go back and too lazy to level up more so I went at it again and barely beat him.