Mega Gardevoir's Attack is terrible by Mega Evolution standards. Physical sets may seem tempting, but they're underwhelming, especially when Mega Altaria does physical Pixilate sets better because of its access to Dragon Dance.
Psyshock is the preferred Psychic STAB on Mega Gardevoir in order to let it pierce through special walls like Chansey. Also, what's the point of a Bold nature when Mega Gardevoir's Defense isn't being invested in? Even with significant investment in Defense, Mega Gardevoir is still frail. Mega Gardevoir is better off with 3 attacks + Taunt, Will-o-Wisp, or Calm Mind. It really likes having extra coverage. The only use of Shadow Ball is to hit Mega Metagross and Jirachi, but you should have other teammates to handle them:
-Psyshock
-Hyper Voice
-Focus Blast
-Calm Mind/Will-o-Wisp/Taunt
Nature: Timid/Modest
EVs: 16 HP/8 Def/232 SAtk/252 Spe
Item: Gardevoirite
Ability: Trace
The above EV spread lets Mega Gardevoir avoid the 2HKO from Latios' Psyshock.
My set has been working fine for me on Showdown, but I'll try yours too. Thanks for the advice.
-Midnight