Perish Trap
Whimsicott + Politoed loops Perish Song; Mega Gengar (Shadow Tag) blocks the switch. 3 turns to victory.
The build
Perish Trap wins by stalling for 3 turns under Perish Song. Mega Gengar's Shadow Tag pins the opponent in; Whimsicott's Tailwind + Encore disables responses. Played correctly it's nearly unbeatable; played wrong it's a quick loss to anti-stall.
Signature Pokémon
Gengar 
Whimsicott 
Politoed 
Charmeleon Lead options
Gengar + Whimsicott for the classic trap. Switch Gengar in turn 1, Whimsicott calls Perish Song.
Win condition
Land Perish Song, keep Shadow Tag up, switch Gengar in/out via Whimsicott Tailwind to stay alive 3 turns.
Hard counters
Taunt users (Liepard, fast Whimsicott), and Ghost types, which cannot be trapped.
Signature moves
Sample team
Plug-and-play 6-Pokémon roster you can import to the team builder. Adjust EVs / nature / move slots based on your specific local meta and tournament reg.
Incineroar 
Garchomp 
Kingambit 
Tyranitar