It's mostly a matter of luck with it. Just lower the health of it and chuck balls at it. Note that stuff like Dusk Balls at night work far better than Ultra Balls so consider taking it on at night time - you can buy Dusk Balls in a few marts (just go to another if one doesn't sell them, although I think Ecruteak is one that does?). Status effects also helps - the best one is sleep (Sleep Powder, Hypnosis, etc) but you need to reset that every time it wakes up. Paralysis works as well as burn and poisoning but is better than those as it does not run the risk of Ho-oh fainting in a few turns.
Good luck! If you have any more questions please ask them in the Help thread sticky at the top of the section. Closed.