1. Build up multiple legions before you send the first one to the front.
  2. If your legions start to get worn down, merge them. Use one as a source of men for another two legions.
  3. If you notice one army is running out of soldiers, recruit reinforcements or a new one and ship them over.
  4. The strategically important cities of the east are Antiocheia, Alexandria, Seleukeia, Babylon and Baktra. That's where they build all their shiny Hellenic elite units. Raid them.
  5. Try to keep your frontline short. Conquer Egypt at one time, than defend it at the neck east of Alexandria. Than conquer Asia Minor, and so you can make a double assault from two directions to their heartlands Syria and Babylonia.