I didn't know Final Fantasy existed until after I beat Kingdom Hearts 1-2, so I'd say the optional Sephiroth battle in KH2 at Hollow Bastion. I was level 73 and had beaten the game already when I saw this weird silver-haired dude looking over the cliff by himself. I had a couple potions/ethers and 1 elixir on me, and some munny so I figured he was selling something.
I walk over there, and he starts on about how nice my keyblade is? Then I got suspicious a bit, and was terrified after he confirmed my fears with these words.
"I wonder if it will change its mind once I defeat its wielder" [draws blade and One Winged Angel starts]
Me:

huh wtf is going on "PRESS TRIANGLE"
Seph: XD Lul
I had the Oblivion key, fully leveled up Valor and Wisdom form, and good blocking sense so my style was to always hang back and read the enemy as clearly as I can to find opportune moments so it took 2 attempts to beat him. He killed me once because of that meteor storm thing caught me off guard. The first time he did that Zen Harvest thing that drops your health to 1 and magic to none was the first time my hands shook as I rushed to the elixir in my inventory. "Crap Crap Crap Crap!"