Since your lead is very vulnerable to special choice scarf users I'd propose running Snorlax instead of Gengar. Combined with Metagross' defensive capabilities your opponent will be forced to switch out, which can be abused with the following Snorlax and Metagross sets:
Support Snorlax
Item: Choice Scarf
Nature: Jolly
EVs: 252 Att / 252 Spe
Belly Drum
Icy Wind
Selfdestruct
Double Edge
combine it with this Metagross to secure an easy sweep
All eyes on Metagross
Item: Salac Berry / Lum Berry
Nature: Jolly
EVs: 252 Att / 252 Spe
Meteor Mash
Psych up
Rock Slide
Earth quake
If you pair these two with a Follow Me support Poke like Blastoise there won't be anything to stop you.