Use hit and run tactics with the horsearchers against Baktria or AS. Be proactive. Once you're at war, don't hide behind walls or in cities. That negates your strength. Send your horsearchers to seek out and engage incoming armies. Empty quivers. Retreat (if you're still too outnumbered to charge en masse and cause a chain rout). Repeat.

With Pahlava, I never bothered to train infantry in the early going aside from garrison duty. Send spies to open the gates. And again, during sieges, empty quivers and withdraw if you have to. Repeat until the numbers are on your side and you can mop up the depleted infantry.

In the early-going, against units like Eastern Skirmishers, Persian Archers, Hellenic Spearmen, Hellenic Levy Phalanxes, etc., a horsearcher unit can take out 100-125 units each battle before you have to retreat.

It can be tedious fighting so many battles, but this tactic is brutally effective. Especially considering how many units the AS can throw at you each turn.

With Pahlava, I conquered up to Persepolis from the AS using nothing but horsearchers and cataphracts. The only infantry units I trained or hired were to keep the peace and defend my newly-conquered cities. Infantry slow your armies down.