I would give it the soothe bell and battle lots and lots. Always keep it conscious and in a healthy condition. When it loses health or gets inflicted with a status condition, heal it right away. And because they're so bitter, never give it the herbal medicine, for that lowers they're happiness. Plus another method is to bring it to Daisy (Gary's sister) every day, possibly after beating the Elite Four, and let her groom it. That will improve its condition, too.