Make sure that Alex has learned "Flash" (at level 18). You should have it by the time you reach Sub-Vane. If you do not, keep fighting the Snow Dogs in Sub-Vane until you learn it. After you learn it, buy two Quick Boots and equip them on Alex. Next, go to your menu, and then to "Tactics", change Alex's Tactic to "Flash", and everyone else's to "Defend". Enter a battle, select "Tactics", and Alex should wipe out all of the enemies on the field.