A good partner I would suggest for this core is scarf Magnezone. It is able to beat out Banded Scizor who could pursuit trap Gengar and 2hko Mega Lopunny with bullet punch. It also outspeeds adamant Talonflame, non-scarf Keldeo, and OHKOs them with Thunderbolt.
Calcs:
252 SpA Magnezone Volt Switch vs. 0 HP / 0 SpD Talonflame: 312-368 (105 - 123.9%) -- guaranteed OHKO
(This is weaker than Thunderbolt, but it gains momentum and allows you to safely bring in Gengar while guaranteeably OHKOing Talonflame in return.)
252 SpA Magnezone Thunderbolt vs. 0 HP / 0 SpD Keldeo: 320-380 (99 - 117.6%) -- 93.8% chance to OHKO
also:
252 SpA Magnezone Hidden Power Fire vs. 252 HP / 0 SpD Scizor: 316-376 (91.8 - 109.3%) -- 56.3% chance to OHKO
(All guaranteed OHKOs with stealth rock)
It's more likely than not going to render Scizor useless, or at least heavily weaken it for Gengar to finish.
A moveset like this should work:
Magnezone @ Choice Scarf
Ability: Magnet Pull
EVs: 252 Sp. Attack/ 252 Speed/ 4 SpD
Bold Nature
IVs: 0 Atk, 30 Speed
- Thunderbolt
- Volt Switch
- Hidden Power [Fire]
- Flash Cannon