Umbreon is a beast against ghost pokemon o.o
anyways yeah jolteon would be a good pic if you don't wanna do the gey friendship thing (which is weird because all you have to do is let it reach lv 21 at night)
Or you can dump a bunch of cash into the stat items until it doesn't accept them anymore, then just level up once. Don't know where Bankai is in the game, but if he can do the Elite 4 and Red easily, he can spam them for cash quickly, then just buy them all at any Department Store.