In my Hayasdan Campaign I made the largest army I could with 14k mnai. It was a mix of Caucasian Spearmen, C. Archers, Armenian Spearmen, and a couple skirmisher cavalry with my FL. First thing I did was use this army to wipe out every Seleukid army nearby, which cleared the way for the army to take Phraaspha and Ekbatana. With this new income I was able to retrain a bit and get a few more units and I marched on Mesopotamia. I started with Karkathiokerta, Edessa, and Susa to encircle the middle of the Sele's empire. At that point I was making enough money that debt wasn't an issue and I could take any city whenever I wanted, which I did and now the Sele's are split in two.
Some things to remember as you go along:
Don't worry about debt. If you win enough you'll make money.
Don't worry about retraining until you absolutely have to. Just merge units and fight all battles yourself. I used basic hammer and anvil tactics against even the elite phalangites and went mano e mano with my FL against their general. In no time he was a 2 gold chevron killing machine with 10 command stars.
Once you have the money, make little relief forces to chase away the small Sele armies that besiege your cities. They usually will send 2-4 units if your city only has an fm or single garrison unit.
I hope this helps! If you need more I would be happy to oblige.