What you need to do is turn on the EXP Share. It also shares the EVs gained in a battle. So all you do is grab a Pokemon that knows a move that can hit all the foes, and train your Cresselia this way.
Another thing you can do is have your Cresselia hold one of the power items. You can purchase them at the battle resort for BP, and they double the EVs gained in a certain stat (Power Band doubles attack EVs for example), and stacks with Pokerus.
Source: I do this myself for my level 1 Pokemon I bred for battling purposes.