I'm still fairly new to Persian Positions, so the explanation about using different troop types was really helpful. At first I assumed the best approach was just to send my strongest units every time, but it looks like troop composition matters a lot more than I thought. 

Hello Archon. As a player to a player, I recommend you to finding an in-game mentor to guide you on how to play the Persian Position. Otherwise, it will take time to become experienced, and it will cost you unnecessary units losses. My best advice is to send units that you can build. The Positions payout doesn't come because you killed the Xerxes forces, but because you sacrificed your own units to Xerxes bank.