The Chocobo Eater in FFX...of course, you don't actually have to win that battle... but at least for me, I couldn't have beaten it at the level I was at.

For raw difficulty, the Great Malboro in FFX. Even with my god-like auto-haste auto-pheonix shields on everyone, I died so many times to this beast (the berserk and confuse prefent the auto-pheonix from working)