What do you give to a Pokemon who has everything? A mega-evolution, obviously. ;P But since Sratagem doesn't have everything, let's give him some defenses.
Seriously, what helps Stratagem's staying power better than doubled defenses? Say hello to Aurora Veil, Alolan Ninetales special, new move. In the hail, brought by Ninetales-A's Hidden Ability Snow Warning, Aurora Veil simultaneously summons Reflect and Light Screen, doubling both the defenses of the Pokemon on the field. That'll keep your Stratagem alive. Seriously. He'll take 2 Meteor Mashes or 3 Bullet Punches from Mega Meta.
But Ninetales-A isn't just for settin' up defenses. She can fight back in her Hail, launching perfectly accurate Blizzards, or hitting those Water-types who hurt Stratagem super-effectively with Freeze-Dry. Moonblast is mandatory on most Fairies who can use it, and Hidden Power Fire hits other Ice-Types or even Steel-Types super-effectively. Does Alolan Sandslash think he can come in on your Ninetales and Slush Rush? Nuh-uh. Not with Aurora Veil up and HP Fire comin' at his face. You worried about special walls? You probably should be. Until you Toxic them.
All in all, Alolan Ninetales helps cover Stratagem's biggest weakness, a lack of defense, and gives him some needed staying power to sweep, while covering his grass and water weaknesses, and having Stratagem cover her fire, poison, and rock weaknesses with his coverage.
Some numbers:
252 Atk Tough Claws Metagross-Mega Meteor Mash vs. 0 HP / 0 Def Stratagem through Reflect: 295-348 (91.9 - 108.4%) -- 50% chance to OHKO
252 Atk Tough Claws Metagross-Mega Bullet Punch vs. 0 HP / 0 Def Stratagem through Reflect: 132-156 (41.1 - 48.5%) -- 9% chance to 2HKO after hail damage
252 SpA Life Orb Stratagem Fire Blast vs. 0 HP / 0 SpD Metagross-Mega: 273-322 (90.6 - 106.9%) -- 43.8% chance to OHKO
252 SpA Life Orb Stratagem Earth Power vs. 0 HP / 0 SpD Metagross-Mega: 224-265 (74.4 - 88%) -- guaranteed 2HKO after hail damage
Aurora Veil gives Stratagem the chance he needs if he's forced against Mega Meta 1v1, instead of him just dying on the spot.
252 SpA Life Orb Stratagem Paleo Wave vs. 0 HP / 4 SpD Charizard-Mega-Y: 608-717 (204.7 - 241.4%) -- guaranteed OHKO
252 SpA Life Orb Stratagem Paleo Wave vs. 0 HP / 4 SpD Ninetales: 343-406 (119.5 - 141.4%) -- guaranteed OHKO
252 SpA Ninetales-Alola Freeze-Dry vs. 248 HP / 164+ SpD Politoed: 132-156 (34.4 - 40.7%) -- guaranteed 3HKO
252 SpA Ninetales-Alola Freeze-Dry vs. 248 HP / 8 SpD Pelipper: 444-528 (137.4 - 163.4%) -- guaranteed OHKO
252 SpA Life Orb Stratagem Paleo Wave vs. 248 HP / 8 SpD Pelipper: 452-533 (139.9 - 165%) -- guaranteed OHKO