In Gen 5, you can EV train level 100 pokeomn because the EVs are applied as earned, not by level up. So you'll need to obtain some of the berries that erase EVs, and get rid of all the EVs your Deoxys currently has. If you have an AR, it is easy to get said berries. Clear all of it's EVs, and then you can re-EV train it in the stats you want. Note that you can't find Bidoof in Black and White 2, I suggest doing it in the swamps of Icirrus City, where Stunfisk and Palpitoad can give you HP EVs.