Get quick,dusk,ultra and timer balls.
Save before you talk. After it's cry, get ready to battle!
Throw some quick balls, if that dosen't work, bring a lvl 51 staraptor with 1 hp, and use endeavor only if speed is high, or bring a scyther and use false swipe. If you both didn't work, try confuse ray + toxic, and get it's hp down to red. Then start throwing ultra balls or timer balls (only if you battled it for a long time). It should work, but if it dosen't, try this: bring a staraptor with endeavor and has 50 hp, then confuse ray+ toxic (golbat is best), then quickly, throw some ultra balls and dusk balls before it faints!