If you have a high-leveled Crobat it will probably outspeed the Ditto-Arceus. What I did was Confuse Ray 3 times and Cross Poison for the toxic poison to wither it down. After Crobat faints just play defensively if you have to. El won't have a full restore and the most the Arceus can do is either sweep your team before it dies to toxic or recover, but toxic will eventually kill it especially if you can land a few fighting moves on it.