Adamant Ditto? Congratulations! :D
Right, good job. Firstly, EV Train it 252 in Speed. This gives your Ditto a lot of speed, but don't bother training it in any other stats, as they all gt decided by the opponent.
I suggest the item as a Choice Scarf, so that you can definitely make sure that you outspeed your opponents Pokemon. This lets you attack (possibly KO) the opponent.
For leveling up, I would use an Exp. Share because training Ditto isn't a quick or an easy feted as such. Once it's level 100, have fun trolling Physical Attackers :D