Surf in the water around the lake near twinleaf town, or fish, and you will find it, but it is very rare, I don't know anyone who has seen it more than once, if you fish, super rod only. I warn you now, it is very powerful, and if actually manage to see it, be prepared to fight a level 70 pokemon, yes, very high level, be careful, but you might not find it, very rare, you need good luck, so, good luck.
