You can trick the AI a bit by luring their armies into your lands, while your towns are very lightly defended. That way the AI will send apropriate, small raiding parties/quoter stack contingents, since your homeland seems undefended. Now, all you need to kill those are 3-4 HA ( Skythians are cheap and available in Mzkheta and Kotais ) + 1 FM ( to charge weakend units and rout them ) + 1 medium cavalry ( to double the charge, deal with Seleucid FMs, since they´re armed with axes = ap weapon, and to cut down routers, since your FMs are too slow and have only "good" stamina ). You can ambush their forces, or you can wait untill they´d besiege your towns. You also can leave this small army in a town, as a garrison, while FM makes money as gouverneur, he still will be leading sallies and won´t get bored, gathering command experience etc. The sally tactic is especially effectiv with the "horse" army i describe above, for couple of reasons:
1. You can sally at least twice before the enemy would build a single siege engine: if you haven´t managed to rout the enemy in the first turn, you simply withdraw into the settlement and wait till battle time is over - it counts draw ;)
2. While you sally, the enemy mostly tries to reform at some place initially different from where they start, so you have "free shots" at their backs, especially at the slow moving phalanx units.

Heroic victories are guaranteed!

Now you are free to go pillage enemy lands, sack their cities, cut their reinforcements, ambushing them wherever you can ( just use the same army composition as described above, and don´t bother with food soldiers - you´d need them only facing really large enemy armies, and for siege purposes. 4 FM in a stack - i never do that, 2 are maximum, 3 only if i face multiple enemy FMs and got faction leader or heir, kind of Roayl Army then.

If you own Karkathioherta, the defence is pure fun - then you don´t need any HA at all, only an FM and enough archers + some spearmen to hole the line long enough ( that´s because of the terrain around the city ). And don´t forget slingers - once Seles start to send elites against you, HA are a waist of time against their armour, even silver chevroned ones, but you slingers would shredder them to pieces if they get a clean shot at their backs!

Hope, this helps.