There are two Mega Gardevoirs so for the moment I will assume you are using the brilliant arrow one.
To make this standard, @Jedidr:
-2 Spritzee
-2 Aromatisse
-2 Super Scoop Up
+3 Fairy Drop
+1 Sycamore
+2 Xerneas BREAK
Fairy drop is nice for making Gardevoir stick around longer, preserving...