Yes, Mt. Battle would work good for purifying. You can also leave another Pokémon at the Daycare to be purified while you are battling. If you have done everything that there is to do, then you may as well sell any unwanted items that you don't need, and don't want to trade with your GBA Pokémon games, and spend your money on the Scents at the Agate Village PokéMart, so that you can work on purifying a little bit quicker if you want.