Azumarill would be a good shout, as it can dispose of pesky rock/ground type as well as negate having to use Charizard X against other dragon types, Mega Charizard X will also help Azumarill by removing bulky grass types, as well as steel types. This is a sample set from Smogon, though I'd just go for 252 speed seeing as the matches in the Battle Maison are fairly short, and that the opponents Pokemon will be random. Also, I'd go for Knock Off over Superpower because Charizard can take care of Steel types, and play Rough cover both Fighting and Dark types, and most opponents Pokemon hold items in the later rounds.
Azumarill @ Choice Band
Ability: Huge Power
EVs: 172 HP / 252 Atk / 84 Spe
Adamant Nature
- Play Rough
- Waterfall
- Aqua Jet
- Superpower/Knock Off
Another good team mate available in AS would be Excadrill. Ground typing +Earthquake will take care of electric types that pose a threat to Azumarill, Iron Head to remove Fairy types. Another sample moveset from Smogon.
Excadrill @ Life Orb
Ability: Sand Rush
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Earthquake
- Iron Head
- Rock Slide
- Swords Dance/Stealth Rock
The only nastiness is that both Charizard and Excadrill have a weakness to ground types, so Azumarills effectiveness against ground types is even more important.
As an alternative, you could go for Skarmory over Excadrill. The disadvantage here is that you'll have a weakness to electric type moves in Skamory and Azumarill with no obvious way to counter those. At least with the 2* weakness to Ground in Charizard X and Excadrill you have Azumarill to counter this.
