Getting a Thunder/Fire Egg
First of all you need to have Eevee and Ditto in the Daycare. You may want to back up your game on the computer if you can. Activate at least 30 Action Replay codes and you should have 5 character change codes.
Whatever you do don't fly while you have all the codes on or your game will freeze. Now your character should look different than what you've seen before. Now go to the daycare and there should be a bad egg in place of Eevee.
From now on you will recieve Eggs that hatch into eggs that you can battle with. To fix your character go in then out of a building then you will return to normal but you will no longer have the national dex and it will look empty just switch it back like would when switching between the heon and national dex.
The Thunder Egg as I call it nows one move ThunderPunch and somehow it will suddenly now FirePunch without you nowing it learned.
More tricks, tips and cheats for this game are right here - Pokemon Emerald cheats