When battling the Ditto-Arceus, most of you here would not want to lose to that "el" dude. However you may not be able to walk the talk due to "some" difficulties. However, from this thread onwards, I can guarantee that you will be able to defeat him if you are willing to get cheeky.
Requirements:
1. Pokemon with Sturdy (at most 20 HP)
2. At least 40 potions to be safe
3. Patience (which those of you who have lost to him a billion times would have by now, being able to defeat him)
Procedure:
1. Using Common Candy, or any way possible, get a pokemon with sturdy and at most 20 HP. Try to level him to one. [The mountain at Apophyll beach has plenty of Geodudes]
2. Before fighting El, put the pokemon in the front, with full HP.
3. Engage in battle, and just keep using your potion every turn. Each potion will heal him up to full health again, allowing sturdy to reactivate.
4. Wear him out, let Arceus use up all his PP. You know when he's running low on moves when he recovers with full HP.
5. Let him struggle. If you want to take the risk, you can let your Sturdy pokemon faint and swap in another pokemon to gain the experience.
The end. Rinse and repeat for possible future single bosses.