After reviewing most of the major battles in the game (I won't spoil anything), I think Gengar would be more beneficial to you. It's STAB Poison typing in particular can help out with a lot of different fights. The big ones that it can help out with (Again, while avoiding spoilers) are the second and fourth nobles, along with a couple of major trainer battles. Although there are certain situations where Golem would be the better pick, I find Gengar to have a better advantage. I'd still use a ground or water type on your team, however, so maybe you could use both if possible?
For a final moveset, I'd use something like this;
Shadow Ball
Dark Pulse
Venoshock/Sludge Bomb
Hypnosis
This game gets really tricky at times, so good luck!
