Your Problem Is Solved. Get A Lot Of X-Eye Seeds Or Sleep Seeds. Use Moves While He is Asleep, or Acting Drunk. You Should Be Able To Hit Him Eventually. Items Dont Miss As Much As Moves. So Use Spikes, Etc. As Dt said. Soooooooooooooo Hope This Helps:)
P. S. If Hes Acting Drunk after The X-Eye Seed, He Should Be Easier To Hit.
Congrats on Beating Mystery Dungeon:)