GliBliss + Mega Kangaskhan.
This is a very effective, if not the best strategy to get win streaks at the Battle Maison.
Gliscor and Blissey cover almost everything defensively, and Kangaskhan has offense covered.
Just lead off with Kangaskhan, and attempt a sweep. If something goes wrong you always have Blissey and Gliscor as a backup.
Eviolite Chansey is also an option, probably better than Blissey actually.
This works best in singles, but is usable in double and possibly also triples, although you need to add some Pokémon.
Here are the sets:
Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 252 HP/ 232 Def/ 20 Spe
Blissey @ Leftovers
Ability: Natural Cure
EVs: 252 HP/ 252 SpD/ 4 Def
Kangaskhan @ Kangaskhanite
Ability: Scrappy ---> Parental Bond
EVs: 252 Atk/ 252 Spe/ 4 HP
All credit goes to the Youtuber Koi and Dragon from this video.