A master ball is a great choice always but personally I would recomend to save it for later. Some other good options are using ultra and dusk balls. You can also try your luck and use great balls (I wasted 4 pokeballs and 7 greatballs on catching my Giratina).
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Just be paitent and catch it with a pokeball!
MasterBall
Any pokeball will work but an ultra ball is probably best.
A Master Ball
try master ball it is the best olviously