Drapion has had some success as a setup sweeper in Battle Maison singles, but not like that.
Typically, it's paired with a Durant, which you can't get natively in ORAS, so you'll need XY for that. Go to Terminus Cave with a Sweet Scent user, and keep calling on horde battles, looking for hordes of Durant. Ideally what you're looking for is one that broadcasts the hidden "Truant" ability on the second turn, which is your cue to eliminate the other four and catch the Truant one. If none of them have that ability, keep trying until you get it.
The point of Durant, and of Truant in particular, is that it gets the Entrainment move to pass that ability onto your opponent. It's an extremely cynical set, taking advantage of the fact that the AI trainers don't realize just how much they've been neutered when their ability has been replaced with Truant, but all the more reason for you to hold that against them. Such a Durant wants near-max speed, even going so far as to have a Choice Scarf to help it outspeed things up to and and including Choice Scarf Manectric, which otherwise threatens a KO with Overheat. It's actually not such a big deal if they do KO Durant, even in one hit, as long as it's been able to successfully use Entrainment first.
Once the opponent does have Truant, you'll want to switch to your Drapion or other setup sweeper (if they did KO Durant on the first turn, this even becomes absolutely free). The opponent will loaf around on turn 2, and now you can get into a rhythm of using Protect on the turns where the opponent will be able to attack, leaving the other turns free for you to use setup moves without repercussion. Shell Smash Cloyster and Dragon Dance Mega Gyarados can both get set up rather quickly, but if you can afford to wait long enough, Drapion is generally superior because it has access to Acupressure, a single moveslot that can max out all seven of its stat modifiers after you use it 21 times. That means max attack, max speed, max accuracy (allowing it to laugh in the face of Double Team/Brightpowder users), max evasion of its own, and of course max defense on both sides. Furthermore, a Drapion that's used as part of this strategy is usually chosen to have the ability Battle Armor, so the opponent doesn't even have the faintest hope that they might get a critical hit to break through those defensive boosts.
While you're at it, you can also set up Substitute, which has the distinct feature that you can use it in place of Protect on occasion, effectively doubling your supply of PP on that move while ensuring that a Truant opponent can't possibly break your substitutes at a faster rate than one per four turns. That milestone is significant, because four turns of recovery from Leftovers (or Black Sludge, since it's a poison type) will exactly offset the HP it cost to make the substitute in the first place, and you can keep up that pattern without losing any health. Eventually you should get into a position where the opponent simply misses a move into your evasion boosts, or your defense is so high that their attack is just too weak to break your substitute, at which point you can run out a stream of nothing but Acupressure for several turns in a row and get set up that much faster, still behind the safety of your substitute. By having the substitute intact when the second and third Pokemon show up, you can be guarded from any nasty surprises they might carry like Fissure or Horn Drill, and that's really what the point of this strategy is about: blocking out as many possible points of failure as you can from the opponent's potential roster.
Of course, the thing about have Protect, Acupressure, and Substitute all at once is that it leaves room for only one damage-dealing attack. Typically that attack is Knock Off, which has the extremely vital selling point that nothing is immune to Dark, and only one Pokemon in the pool of sets that Maison opponent can have (Scrafty) goes so far as to double-resist it. It also carries pretty reasonable power, comparable to a STAB Earthquake, when it's knocking off an item, and the fact that it knocks off the item is itself a useful asset for getting rid of stuff like Quick Claw and Custap Berry and preventing those from being used as potential vectors for breaking through you. It can also be used early on against an opponent who carries a choice item and who has locked themselves into a low-PP move, making them more inclined to stick around longer with the wonders of more moves to choose from. Even if you have to use Knock Off a few times into an opponent who resists the move, the fact that you're starting with +6 in attack and defense (not to mention evasion) means you generally bust through those foes over time anyway. Remember, a not-very-effective hit at +6 is equivalent to a super-effective hit at +0.
This isn't the end-all of infallible strategies by any means, though. It's pretty tough to stop if you can get it all set up, but there are ways that the opponent can stop you from setting up. The first potential point of weakness is if they can stop you from using Entrainment: an opponent with Magic Bounce, for instance, will use that move right back at you and cause your ability to be overwritten with Magic Bounce. The only potential Magic Bounce user in the pool is Espeon, which you can handily OHKO with Durant's X-Scissor, but if you have a Choice Scarf that's going to mean locking yourself into a Truant X-Scissor, a situation that probably entails having you switch out and back to avoid being crippled yourself. An opponent with Protect can also block the Entrainment for that turn, forcing you to wait until turn 3 to try again, a luxury you might not always have (and they might just wise up and Protect again on that turn). Beyond that, a Prankster Taunt user is one of the few things that can outspeed and prevent you from using Entrainment at all--a situation from which you'll absolutely want to switch, as staying in will only force you to use Struggle.
Other than that, once you're in against a mostly passive Pokemon whose ability has been changed to Truant, the general rule is "they won't switch out until they run out of PP they can possibly hit you with", and in this respect Truant is even more helpful as it makes their supply of PP last twice as long. But some Pokemon will have moves that they can use to take themselves off the field, such as Perish Song, Explosion, or Baton Pass. (Volt Switch and U-turn can do the same, but at least those effects can be blocked with a timely Protect on your behalf.) The AI opponents don't realize that this is what they're doing, but even an Explosion that misses into an evasion boost is helpful to them, as it lets them replace the neutered Pokemon with something that doesn't have Truant, which if nothing else denies you the ability to guarantee the risk-free setup of alternating Protects. They may also have Roar or Whirlwind, which can force you to switch out and lose your boosts, even from behind a substitute and even if you use Protect, unless you deliberately kill off both of your other Pokemon beforehand and leave Drapion as the last one left. Certainly a risky gambit, but one that might be more convenient than the alternative. Finally, as a last-ditch effort, once you've set up completely and finished off the first opponent, the other two (who will have non-Truant abilities) might be able to slow you down just enough with those abilities, in particular Flame Body. This has a chance of causing a burn when you hit them with Knock Off, which not only reduces your attack stat to the equivalent of +2, but also adds passive damage each turn that outpaces your Leftovers or Black Sludge. If they have a defensive Pokemon with a recovery move, which is capable of handling a barrage of your now-reduced Knock Offs, even though they might not be able to break through your own defense and evasion, the fact that you're losing health to burn damage over time means they can eventually win that war of attrition without ever having to hit you.
The Sun/Moon Battle Tree still features an AI naive enough to fall for stuff like this, but there's a few mitigating factors that have been introduced. Most notably, several new Pokemon like Komala, Wishiwashi, and Mimikyu have abilities that are specifically immune to being overwritten by Entrainment or any other means, so you'll have to find other ways to break past those. Opponents now have mega stones and Z-crystals included in the pool of sets they can have, and those items can't be knocked off, so your Knock Off will only ever hit for the weak version (comparable to a Bite) against those enemies. And if the opponent does use a Z-move, that can hit for some damage even through your Protect, so the setup isn't quite as carefree as it was before. There's also the fact that neither of those Pokemon is native to Alola, and you'll have to send them forward through Pokemon Bank, but that's not really much different from having to transfer a Durant for XY to ORAS.
