Gardevoir @ Gardevoirite
Max out your Cleverness.
Heres the idea:
Go for Calm Mind to get yourself pumped up, before using Stored Power. If Lisia mega evolves her Altaria and gets all the hearts, use Draining Kiss to copy her heart count. Use Destiny Bond on the last turn.
This is in the Cleverness category of the Contest Spectaculars. Hope that I helped!