The easiest way to catch "Roamers" without using a master ball, in my opinion is to whittle down it's health and try to get it so sleep, each on individual encounters.
Once it's on low health and preferrably asleep, keep using quick balls each time you encounter it, and you'll get it before long.
I caught Thundurus after about 8 Quick balls. You could also save, use 5 or so, then reset so that you don't lose any :)
Best of luck
Yupp do what chris says
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