If you're going to use it for competitive play I would breed one instead.
Honestly, if you're using the galvantula competitively in Black2 then there's not much competitive stuff that happens there any more, so good luck! xD
if you're catching it in Black 2 and then transferring it to XY because you dont have ORAS then just get a Joltik off the GTS and change the nature via breeding.
Honestly it's a TERRIBLE idea to use caught Pokemon competetively, its much better to breed them and honestly a lot more viable and easier.