We all know that Alain is a Charizard-enthusiast, he used Metang-Metacross very rarely on-screen and the others only in the final. Potentially he could use the other mons, but the focus was rightly on Ash's Team and Alain-charizard duo in that moment.
But still my point remain, even if I really liked the battle in general, Lysandre was basically overwhelmed and back then Malva would have been
perfect to "reinforce" the Team Flare. No, instead Lysandre VS rest of the world. At least Malva... instead to battle Lysandre when she arrived... why not the twerps? I would have even liked a
direct use of Z-2 (in battle i mean)....
I agree with
@shoz999, they simply didn't want to drag further the team flare arc for various reasons (time-consuming to make these episodes, need focus on main cast, etc.).