I remember heavily relying on Chopper and Luffy - but maybe that's just because these two were the ones I liked best.
I've heard a lot of different tactics/tips though, so I guess it doesn't really matter what you do. Do whatever you feel comfortable with; focus on the character you can fight with best.
I just took some revivals with me (they were awfully easy to make in UC 1) and used as many special moves as possible. That's where Chopper really came in handy. I think he was responsible for 50% of the entire damage.
It might take a while until you finally beat the bastard, but I'm sure you'll manage with the help of revivals and stuff like that. You're BOUND to beat him at some point. Every time you lose just reset the game so you'll keep the powerups/revivals/meds you used during the fight. Make some more meds, then try again.
I clearly remember when I finally beat him for the very last time (aka at the highest difficulty). The sun had just risen after a 19 hour gaming marathon; I was exhausted, a nervous wrack and shaking like crazy. It was so goddamn awesome.