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)
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