I caught him with a Master Ball. Was planning to keep it for either the Legendary Bird or something else but he was much harder to catch then I thought it would be. Maybe if you False Swipe+T-Wave him or something then it wouldn't be super painful to catch him with a Dusk Ball. But either way just beware he is MUCH harder to capture then Yveltal/Xerneas/Zygarde who all make it seem easy as heck.
Spoiler
I'm thinking you can Spite all of his Recover away, then False Swipe + Spore for the win.
I REAAAAAAAAAAAAAALLY want my Parasect from Black 2 now :<. It made catching things so easy.
I guess I can wait until December... not like I need Mewtwo right away.