REGISTER TO REMOVE ADS
Ever since I fought Larxene 1 (I'm only on th eighth floor of Sora's Story x|) I ended up getting the sh*t out of me on every boss and started freaking out every time I lose to a boss. I swear, she laid a curse on me. So every boss from LArxene and then on is the hardest.
In all seriousness, the hardest boss was actually the Trickmaster in the R/R mode.
The predefined deck Riku got was just crap on that level. It took me about 6 tries to kill him.
Vexen 2 (totally didn't see the freezing sword of vengeance coming)