There is no "easiest way" just do the normal way. Normal way: Talk to the old man in Pacifidlog Town. Once he see's it go to route 130. Look for mirage island. Note: Mirage island is not that important it only has the rare Leichi Berries and in the grass all you will find is wild Wynat.
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The only way to get to mirage island is by having the the old man in the lowest right house say he sees the island, then just surf left. =D
You have to have a cheat. (For example a game shark).Or you have to go to a Pokemon convention and they will give you a ticket to the island.
On The Second Floor Of The Mirage Tower, There Are Cracks In The Floor. To Cross Over The Cracks, Aquire A Mach Bike From The Bike Shop In Mauville City. Even With The Mach Bike, You Will Fall Through The Cracks. The Only Way To Cross Over The Cracks Is To Gain Speed Before Crossing Over. It Should Take A Few Tries Before Crossing Successfully.
no way to get it
First you need a Pokemon that knows surf. Then surf all the way up from Rusboro City to an island, not the one where battle girl is located.