Xaldin. Because it was the only battle with challenge in normal mode (I'm hoping to change that when my FM + comes). He was the only guy that came really close to killing me, ridiculously close. I didn't know wtf learn was for, and I kept getting hit by his fly-out-and-blow-uber-big-wind-in thing. Mickey saved me 3 times, god. That one I was pretty much expecting to lose more than 80% of the time, especially when I realised that when I came close to finishing him my Berserk Charge was on, and he flew into the background, blew wind, and killed me one more time.
Otherwise will be the card guy, forgot his name, in TWTNW. Quite some challenge, and some luck and quick reactions to not get turned into a dice and get whacked a dozen times, and especially frustrating when there's only seconds left and your berserk charge is active and he has the last health.