Q&A for How to Get Ditto in Pokémon Emerald

Personally, I would suggest Rayquaza. It's easy because it's already level 70 when you catch it. Guardevoir is a good choice too since it can sweep through most of the Pokemon (not counting Ulrich, the dark type) and can help you against the ghost type member.

No, only through trading over editions or by using cheats. If you are playing on emulator and don't want to use real cheats, vbalink should work.

Yes, it is on a mirage island.

On Route 111 (the desert) there is a tower that is sometimes there and sometimes not. Once you climb the tower (you will need the mach bike to get over cracked ground) you will find the Root Fossil and the Claw Fossil. Pick the one you want, then take it to the Devon Corp. in Rustboro. Talk to a scientist on the second floor, who will ask you if you want to bring it back to life. Say yes, then wait.

You can rematch the Elite Four or Gabby and Ty, as trainer battles yield more experience than wild Pokémon battles. According to Bulbapedia, you can find Gabby and Ty on Route 111, Route 118, or Route 120. Use these strategies along with the Exp. Share to gain a lot of experience for your Pokémon.