Double Dark Mew

Did... You just say to transfer Mew?

It actually isn't possible to transfer Mew. The more you know! 😁

Shadow claw would be even better against Mewtwo, but he could easily burn through all of his TM's, and end up with something way worse than snarl. So, I agree, keep the two dark moves. That's probably one of the better movesets Mew can get.

You were extremely lucky, then. I spent seven charge TM's on a karate chop Machamp and still didn't get DP. It took me six charge TM's to get shadow ball on my shadow claw Gengar, and five to get heavy slam on a Snorlax. With Mew, you could easliy spend more than 50 TM's without getting the moves you want.

21 Fast TMs is more than enough. You can change 21 non-Mew Pokemon to the desired fast move. When do you ever have a moment where you need to fast TM 22 Pokemon (in a relatively short moment, as you get new ones quite fast) anyway?

I agree with the rest though. If you are going to power it up directly spending TMs on it may be worth it, otherwise you can better keep those TMs if they're not bothering you.