i just level up my forms at the mansion in twilight town.

Start from the white room, i change into the final form (don't change at places where there's battle, u might change into the anti-form instead) and just rush in the mansion, killing all those nobodies, and stop at the computer room's save point, barely got enuff drive left to go to world map and retain a full drive bar. repeat until level 7. (Total=26 exp per trip)