It's more than clear that you have reached the stage of one of the most frustrating stuff to do in the game due their enormless hax level and disturbing use of status moves. But fear not, the lord shepherd shall guide thee
First of all, let me introduce you to the wonder move in Substitute
Substitute is one of the most divine moves in the Battle Maison and you should thank Lord Wooper for giving so many Pokemon access to it. Due to the high usage of moves such as Toxic, Will-O-Wisp, Thunder Wave (I'm looking at you Tyranitar e.e, etc.) having substitute on one Pokemon or even all 3 of them will do you more then good in most battles
The move is great in many different ways as it shields you from the cheapest strategy that maison has to offer you, OHKO moves...
Take my word for it, the last thing you want to face is someone using stuff like a Quick Claw Sheer Cold Walrein on your 99th winstreak ;~;
Substitute also helps you get passed the odds of Quick Claw any move and the usage of Focus Sash on the most random Pokemon.
We've all been there before, that moment when you say, let me use Hydro Pump over Surf, because it will enable the KO on that Pokemon and that Pokemon and will prevent Ebola from sweeping my nation? Ye, no. The Battle Maison does not know mercy and for some odd reason, if you use Hydro Pump in that haunted place, you will never hit it, so please, I beg of you, even though the power might seem tempting, if you have access to a more reliable alternative to certain moves like Thunder, Blizzard, Fire blast etc., for the love of Wooper, use them.
You will thank me for this.
Offensive Boosting+Supereffective nerfing berries
Don't do stuff like slow calm-minding in the battle maison, they will just end up critting the living day out of your innocent soul. Instead, despite this seeming a bit odd, slap a supereffective nerfing berry on your set-up sweepers. DD Haban Berry Haxorus is a great example for this. Not only does it wield great power, but it also enables you to do some emergency setting up on opposing dragon types and then proceed to sweep their evil teams.
Out of experience I can tell you that, if you limit their space to attack, you are more than likely to plow your way through the battle maison. Isn't that lovely.
There isn't much to say about this. Lord Wooper personally gave you the ability to Mega evolve Pokemon and deprived them of their right to do so in return. Abuse this privilege and claim your title as Master of the Maison.
Attack drawing abilities
Due to the AI not being able to think as rationally as an actual human being, abilities such as Storm Drain and Lightningrod work exceptionally well, especially in super doubles and super triple battles. This will make certain Pokemon nearly invincible in battle and generally enable you to set up freely.
A prime example of this is using Gyarados and Mega Sceptile. Mega Sceptile's massive speed and special attacks as well as its ability in Lightningrod will commonly leave your opponent powerless against Gyarados, who can now freely set up Dragon Dances and commence to sweep the opposition.
Recommended Pokemon (singles)
My dear Padawan, if there is one Pokemon I recommend at all, it should be Mega Kangaskhan. With her great ability, massive bulk and attack and charming curves , she will bring you great joy and hand over to you the heads of your enemies. The fun thing about Mega Kangaskhan is the simply fact that she breaks surprise sashes and has a certain proclivity to KO every possible Pokemon you can potentially run into in the battle maison. The best bit should be the fact that its weaknesses can easily be covered by teammates.
Aegislash is another Pokemon that is still fantabulous in the Battle Maison, due to its share bulk, Arceus stats and great offensive prowess. It is one of Mega Kangaskhan's favourite partners in Battle Maison crime due to it flawlessly covering its only weakness and does a great job at hitting / weakening Pokemon till the extent that the only thing left for the other 2 Pokemon to do is clean the remains. Like in the actual competitive world, SubToxic Aegislash is a fantastic Pokemon to use in the battle maison and in conjunction with King Shield can make life very unpleasant for the AIs you have to face. Generally, special Aegislash is a Pokemon you definitely want to consider using in the Battle Maison
Talonflame is one of the last Pokemon I will be recommending, as its Choice Band set is just fantastic. The best thing is, you don't even have to worry about dying because its main job is usually to just punch massive holes in the opposition and then have the likes of Mega Kangaskhan to rip the remainders apart
Sylveon is an excellent Pokemon in pretty much all 3 facets of the battle maison. When running the Choice specs, or Life Orb, the only move you will ever be using is Hyper Voice. It will easily KO everything that does not resist this dreadfully powerful attack. It should be noted that Sylveon shines the most in super doubles and super triples battles.
The listed Pokemon are just Pokemon I recommend trying out as they are known for making the Battle Maison a little easier to handle