252 SAtk > 252 Def, as you have the Sash. This one is more optional, but Taunt > HP Fire might work nicely, as instead of killing leads, you can force them out and get more Spikes up. Other than that, it looks fine.
I can see what you are going for, but there are a couple ways to improve it.
1. Add Substitute, Gengar doesn't have enough bulk to use Pain Split without it.
2. Use Life Orb so you can recover more HP with Pain Split and hit harder.
*Gengar @ Life Orb
EVs: 252 SAtk / 4 SDef / 252 Spd
- Pain Split
- Shadow Ball
- Sludge Bomb / Focus Blast
Leftovers > Expert Belt to stay around longer, as Golurk is a bulky attacker. 252 HP > 252 Def as Golurk makes a fantastic mixed tank.
I recommend replacing this for Sableye. Not only is it neutral to dark, which wrecks this team, but it has a very similar movepool, with Night Shade, Will-O-Wisp, and Taunt, all of which are gotten off faster, courtesy of Prankster.
*Sableye @ Leftovers
EVs: 252 HP / 252 Def / 4 SDef
- Night Shade
Hope this helps and good luck! ;)