Gengar-Mega @ Gengarite  
Ability: Shadow Tag  
EVs: 252 SpA / 4 SpD / 252 Spe  
Timid Nature  
IVs: 0 Atk  
- Moongeist Beam  
- Sludge Wave  
- Focus Blast / Icy Wind  
- Taunt / Substitute / Destiny Bond
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Optional Move(s)
- Acid Spray
- Hidden Power [Ice]
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Gengar-Mega @ Gengarite  
Ability: Shadow Tag  
EVs: 252 SpA / 4 SpD / 252 Spe  
Timid Nature  
IVs: 0 Atk  
- Hex  
- Will-O-Wisp  
- Sludge Bomb / Sludge Wave   
- Taunt / Substitute  
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Optional Move(s):
- Toxic Spikes
- Baneful Bunker
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Overview:
Mega Gengar's Shadow Tag ability and its high speed lets it selectively trapped opponent it wants to. Its unique typing provides it a useful utility both offensively and defensively, granting it a great STAB combination, an immunity to Toxic and the ability to remove Toxic Spikes on the field. Despite this, its low bulk gives it a hard time to switch into or safely Mega Evolve on sets without Substitute or Baneful Bunker. Defensive teams are aware of its presence by packing a dedicated Pursuit Trapper or using Shed Shell on pokemon it tends to trap.