i want to catch him
i would try to use a quick ball at the start of the battle so he does not transform. once he transforms, he gets harder to catch. weaken it, and get a pokemon that has a move or ability that inflicts a status condition.(freeze sleep,or paralyze you do not want to poison or burn,because it will eventually kill it) and just start chuckin' ultra balls.
Easy - Start with a Pokemon that have high catch rate like Rattata or Pidgey, and when it transforms, just switch to a Pokemon how can weaken it and catch easily.
Use any pokemon that can paralyze it like a Bedew.So when it transform i will be easy to catch since Bedew is a basic pokemon and have a very high catch rate hope this helps :}
never ever bring a legendary to him! get a magikarp so it can only use splash and keep throwing ultra balls or dusk balls at night.