Shadow Claw for Gengar?

Trade or time machine.

It was available in the first 6 months of the game. The only way to get it now is trade.

Get the Time Stone from Thanos

It's a legacy move. Pretty much, your best option would be to get lucky if someone's willing to trade a SC Gengar and it becomes very good. And no offense, but to be honest, I'm not sure why new trainers (well, people that know about Gengar's movesets) would want Shadow Claw anyway. Lick is actually more preferred than Shadow Claw right now. But don't get me wrong. Shadow Claw is still a good fast move for Gengar, but Lick seems like it'll take the cake.