I use a very uncommon strategy that takes time (it works but people are lazy and don't do this). First, I track it down. Then,I lower its health into the low yellow zone with a strong move (can never be too careful). Then, as I continue to track it down I use Ultra Balls. It works. I caught Raikou and Tornadus with this strategy. But,once you use this stategy it won't work on another roaming pokemon i.e. do this with Raikou you might wanna use your master ball on Entei. I reccomend using your master bal on Virizion because it knows Giga Drain and as soon as you get it to a red zone,It uses Giga Drain and you have to do it all over again.