Normalize-Trap Mega Gengar Sweep
To begin the setup, Gengar must first Mega-Evolve to change its ability to Shadow Tag, which prevents opponents from switching out. The rest of the team besides Delcatty have Skill Swap and Entrainment in their moveset, giving Delcatty a 3/4 chance of using either one with Assist, which then transfers Delcatty's ability, Normalize, to the opponent.
When Mega Gengar returns, the opponent is now trapped and unable to hit Gengar because Normalize makes all its attacks Normal Type. Gengar can then freely setup with Substitute and Nasty Plot.
The rest of the team have other moves that can't be used by Assist, giving them some extra utility. Chingling is used as a sack to give Delcatty a safe switch-in. Chimecho and Audino have Red Card to force unfavorable opponents to switch out, and Trick/Covet to remove the their items. Smeargle has some K.O. potential with Counter/Mirror Coat and Destiny Bond.
This strategy only works in National Dex Anything Goes, as it requires Mega Gengar to have Nasty Plot, which is only available in 8th Gen.
Gengar-Mega @ Gengarite
Ability: Shadow Tag
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
Substitute
Nasty Plot
Sludge Wave
Shadow Ball
Delcatty @ Choice Scarf
Ability: Normalize
EVs: 252 Atk / 252 Spe
Hasty Nature
IVs: 0 HP / 0 Def / 0 SpD
Assist
Sucker Punch
Thunder Wave
Zen Headbutt
Chingling @ Rocky Helmet
Ability: Levitate
EVs: 252 SpA
Hasty Nature
IVs: 0 HP / 0 Atk / 0 Def / 0 SpD / 0 Spe
Skill Swap
Entrainment
Endure
Snatch
Chimecho @ Red Card
Ability: Levitate
EVs: 252 HP / 4 Def / 252 SpD
Calm Nature
IVs: 0 Atk
Skill Swap
Entrainment
Endure
Trick
Smeargle @ Focus Sash
Ability: Moody
EVs: 252 HP / 4 Def / 252 Spe
Timid Nature
IVs: 0 Atk / 0 Def / 0 SpD
Entrainment
Skill Swap
Destiny Bond
Mirror Coat
Audino @ Red Card
Ability: Regenerator
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
Entrainment
Skill Swap
Covet
Dig
Special thanks to Mr. David for the music recommendation.
Music: Adagio in G minor by Tomaso Albinoni