You could always duplicate a Pokemon that's holding a Master Ball so that you have two and can spare one. Otherwise, try to get its health down as much as possible, try to put it to sleep or perhaps even try to freeze it (Poisoning it runs the risk of the poison KOing it before you're successful at catching it). Have lots of Ultra Balls AND Heavy Balls.
By the way, to friendly_dragon43's signature, I've applied to and have not been accepted. I don't think she's checked her site in a while.
I am training to be a Glitch Master, baby!