Mewtwo

Mew2 has beastly stats as is so that 12 attack doesn't make a huge difference compared to a perfect 15. If you have a friend with a mew2 he's willing to trade, you have 2 days to do it.

I think with mew2 having some of his moves made legacy and the birds having their exclusive moves this week, we will be seeing some new moves for him. Possibly his signature psystrike?

I'd hold onto the 12 atk, with how powerful shadow ball is, even a 10/10/10 mewtwo would be work holding onto in my oppinion

It's Mewtwo, IVs are less of a concern with him. Currently my only powered up Mewtwo is 11/11/13 a/d/s because for many months it was my only one. Still hits like a truck.

Your current Mewtwo is fine. 12 attack is decent.
Unless you can force a guaranteed lucky, chances of getting a better mewtwo that the one you have are really low. If you're close to "best friends" status wait until then.

I missed the the news... Is Shadow Ball becoming legacy?