KH1: 1st Ursala(didn't know you had to finish/face her with magic), Dark-form Riku, Kurt/Clock Tower/Sepheroth(They were all secret, so DUH they're hard)
I found Maleficant easy......
KH1.5 or CoM: The Dark Riku second battle and Vexen second battle.
KH2: faced Seph 2 time and haven't depleted his health by a full bar. Xigbar(had to get help from the king twice, but won first try), The final part of the final battle(didn't lose, but it put a lot of presure on me to save Sora....)