Since I mostly play Healer in RPGs, Sharla was my character I used. But since you're using Shulk, I'll have to switch up the strategy. When the battle starts, go after the little Mechon. Use Enchant frequently so Reyn can deal damage. Don't worry about Sharla, that's what Thunder Bullet is for: Ether Arts do normal damage against Mechon, and Sharla isn't built for offense. Get the bonus effects