Begin a custom battle and have 10,000 Denerii. Choose a team that has Onagers, or at least Heavy Onagers. Fill one row of units with Onagers. Fill the rest with strong infantry (for example, three Armored Elephants, three War Elephants, and three Normal Elephants. During the battle, use your row of Onagers to fire at a piece of wall. Note: Activate Fire Ammo for better results. Wait until the wall is destroyed. Use your infantry to take out the men inside the city. Do not forget to take out the archers.