Battle of Dhi Qar

Hold the city in the middle. Leave some units near Trapezus to defend the coast from enemy reinforcement. Killing the first Infantry gen you encounter will give you and your allies a Morale boost, which will help you to capture the city in the middle. Let your archer gen in a Horse archer and Mithridates deal with the enemy general guarding the city, and let Arminius circle the city to attack enemy units, and only let him attack the city when you can safely kill the enemy general. Once he's dead, all enemy units will be demoralized, and you should take advantage of it by killing all units that can capture the city, and march towards Amaseia. You can go to the city below instead and take the free 2 scrolls for archer commander. Just surround it and keep killing enemy reinforcements until you kill 25 units, and capture Amaseia. Key is to keep Arminius healthy. Pompey will be supporting him when killing enemy units. Just don't let the enemy reinforcements land by blocking the coasts, and this campaign shouldn't be too difficult.