The area I found the best is the stairway area just past where you fight Xigbar. There's a save point before you fight Xigbar. Start there, run past the snipers in Xigbar's room and go to the next area. At the bottom, there are a few Assassins and Creepers. Halfway up, there's a horde of Creepers, at least 30, probably more. They are easily dispatched with magic, and even at low Final Form levels there's ample time to kill most of them if you enter Final Form halfway up the stairway. When your drive gauge gets down to low 1, just run/glide back to the save point, and exit to the gummi ship. It didn't take me long at all to max out Final Form here.