EDIT: Kill him and face the Pokemon League to get him back brand new and follow these steps. It will be SO MUCH easier.
P.S. this works with any legendary you are having trouble with (except Giratina, being a Ghost-Type and all...)
Pokemon with VERY HIGH attack that can learn False Swipe.
Pokemon that can learn a paralysis-inducing move. (any pokemon that can learn Thunder Wave) Not sleep because that wears off. Paralysis does not.
NEVER Poison, Badly poison, Burn or Confuse the Pokemon. It might kill itself.
Zangoose (M) @ Silk Scarf
Adamant (+Atk -SpAtk)
Role: Bounty Hunter (Not for battle, competative or otherwise)
- Swords Dance (+2 Atk)
- False Swipe (Take them down to 1 Hp without killing them)
- Double Team (to avoid attacks and use less potions)
- Disable (stop annoying moves)
Try Dusk Balls at night. They have a better catch rate (3.5x) than an Ultra Ball (2x).
NOTE: Ultra Ball has 2x catch rate ANYTIME. Dusk Balls only have a 3.5x catch rate at night, and 1x any other time (same as a Poke Ball). So, HUNT AT NIGHT!!!!