Battle Mew
Battle Mew with this 4 line code. You'll basically encounter this Pokemon straight after you enter the code.
AE45D79E2DF6
0686AA5A3903
AE45D79E2DF6
805C1D31C295
Now when you do this, go outside any building that you are in and the press L and Select at the same time and you will go into the battle with Mew (You do not have to be in grass, water, etc.).
If you need more help, we've got more Pokemon Emerald cheats and also check out all of the answers for this game
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