It's worth a shot but perhaps Xerneas BREAK and Mega Gard (Brilliant Arrow)? I mean, in certain cases, Gard can hit harder than Xerneas in the damage department, and even without Mega Turbo, Gard benefits from Geomancy. Maybe do something with Max Elixers too for before Gard Mega Evolves? Maybe possibly a Sylveon GX tech? I'm just spitballin' here, but I could work. Then again, I don't have the best experience with Megas, being that I play Volc/Turtonator, but I've seen Megas go about in their days.