An option that easily beats Gengar is Tyranitar. Tyranitar can also set sand for Excadrill, letting it sweep much more easily. Tyranitar tanks three Dazzling Gleams from Gengar while trapping it with Pursuit or OHKOing it with Crunch.
Tyranitar @ Leftovers / Lum Berry / Chesto Berry
Ability: Sand Stream
EVs: 252 HP / 4 Atk / 252 SpD
- Crunch / Pursuit
- Stealth Rock
- Thunder Wave / Toxic
Pursuit will let you always eliminate Gengar when Tyranitar comes in because it can still hit if it tries to switch out. You can use Crunch if you aren't worried about it switching out. Earthquake is useful as further coverage, and Stealth Rock can be used to break sashes and provide more chip damage. Thunder Wave is useful for slowing down your friend's team, while Toxic can cripple their defensive Pokemon.
Leftovers is a good option to provide Tyranitar with passive recovery, but if you're worried about Hypnosis use Lum Berry. Chesto Berry is an option if your friend is using other status moves, as Chesto Berry doesn't effect non sleep status so it guarantees a cure of Hypnosis.
4 Atk Tyranitar Crunch vs. 4 HP / 0 Def Gengar-Mega: 270-318 (103 - 121.3%) -- guaranteed OHKO
252 SpA Gengar-Mega Dazzling Gleam vs. 252 HP / 252+ SpD Tyranitar in Sand: 102-122 (25.2 - 30.1%) -- guaranteed 4HKO