I used a complicated strategy. this might be long:
Death-Song@gengarite
ability- cursed body--->shadow tag
sludge wave/ bomb- useful, STAB, poison chance
dark pulse- coverage, flinch chance
Shadow ball/ Hex- STAB, good damage or great damage with status conditions
Perish song- obviously
Blissey@leftovers
Soft-boiled/ heal pulse- support or extra sustain
Counter- to attack
Skill swap- you'll see
Stealth rock- reasons
Strategy goes like this: You mega evolve Gengar, then skill swap his ability over to blissey. Boom, now ya' got a serene grace Mega Gengar, a shadow tag blissey, it's perfect! you could set up Perish song, or sweep with Gengar, then set up perish song. I also brought a Soundproof Kommo-o, so if you decide to sweep first, switch out blissey for Kommo-o, perish song. then they can't switch cause they got no more Pokemon, and you can Z-Clanging Scales.