Power up the Gengar with Shadow Claw. Any Gengar with Shadow Claw will perform better than a perfect one with the current best moveset, Hex + Shadow Ball.
I wouldn't bother with TMing the dark Gengar if you can solo Gengar raids. Sure you may be unlucky at the moment but with enough tries you'll get one with just as good, or better IVs and with a better moveset, preferably with Shadow Ball as it is easier to get fast TMs than charge TMs, and unless you have a legacy move, a fast TM 100% gives the other move, whereas charge TMs only have a 50% chance.