In my opinion, the first thing you should do if you wanna get a Ditto, is get it from dream world, so you can get the ability impostor. This allows for an automatic transform, which means you dont have to waste a turn using transform. The next thing I would suggest is just to constantly battle the elite 4 over and over again with Ditto holding EXP Share. And then you can also EV train Ditto, but the thing is once you transform, all your stats change, so there really isnt a point to EV training ditto. Heck if you have a impostor Ditto, there is no point to even train it at all, considering that once you switch into Ditto, all stats will be copied meaning that it is pointless to train him.