Standard Rain Set (Gengar) @ Leftovers  
Ability: Levitate  
EVs: 96 Atk / 160 SpA / 252 Spe  
Hasty Nature  
- Thunderbolt 
- Ice Punch  
- Hypnosis
- Explosion

Other options: Hidden Power [Grass] / Fire Punch / Will-O-Wisp    

Focus Punch + Boom (Gengar) @ Leftovers  
Ability: Levitate  
EVs: 184 Atk / 124 SpA / 200 Spe  
Hasty Nature  
- Thunderbolt / Ice Punch
- Focus Punch  
- Hypnosis / Mean Look
- Explosion

Full Utility + Boom (Gengar) @ Leftovers  
Ability: Levitate  
EVs: 248 HP / 60 SpD / 200 Spe  
Jolly Nature  
- Hypnosis  
- Will-O-Wisp  
- Mean Look  
- Explosion  

Perish Song Trapper (Gengar) @ Leftovers  
Ability: Levitate  
EVs: 248 HP / 60 SpD / 200 Spe  
Timid Nature  
IVs: 0 Atk  
- Mean Look  
- Perish Song  
- Taunt  
- Destiny Bond  

(This set needs to be paired with Substitute Dugtrio (Substitute > Rock Slide in Choice Band sets))

Smeargle Gar (Gengar) @ Leftovers
Ability: Levitate  
EVs: 252 Atk / 4 SpD / 252 Spe  
Jolly Nature  
- Explosion  
- Mean Look  
- Taunt  
- Confuse Ray / Brick Break