How to Catch Deoxys on Pokemon Emerald?

In order to get Deoxys, get a ticket to ride a boat island. On that island there will be a triangle, and you’ll have to do some sort of puzzle. Save your game after you’ve done this. Examine that triangle so that you can bring forth the Deoxy. Just make sure your characters are at least L30 to L40, and then use action replay max in order to get 99 master balls, so you can then use it on the Deoxy. Hope that helps!