I've got Mespirit to a low HP and I've got the easiest way ever to catch but, I need to catch it for another trade to black and white.I've got Uxie and Azelf along with 9 or ten other legendary's.Leave the best answer.
Well, Quick Balls are fairly nice since it is roaming.
Dusk Balls make an excellent choice at night.
Ultra Balls are obvious, but I suggest you use a Mean Look + False Swipe combo.
And the last one of course, Master Balls always work.