Yeah the optional bosses were definitely the hardest, though storyline wise, I probably had the most trouble with the final Riku fight. Though that fight is fun as hell, so besides the god-awful long cutscene before the fight I never minded losing to him.
And Ice Titan was easy once you figured out you could go into the stands of the stadium for that fight. The more distance between you and Ice Titan, the easier he is.
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"It's a magical world, Hobbes ol' buddy...let's go exploring!" - Calvin
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