Charduck you are incorrect, the Dragon Group isn't affected by Plusle/Minum
This is what I used to catch it:
Be prepared, free hands, relaxed, etc.
Ok, when you challange it, don't let it to go back to the floor (capture it on the air, on the floor it will shoot Dragonbreathing) and try the fastest you can, because if you are slow taking it, it will fall and attack with a all-field attack.
Good Luck with it.