Yep, about 9 units of Horse Archers is very hard for the AI Seleucids to beat, at least until they start fielding large amounts of Phalanx pikemen and cav. An all-HA army can also be used to take towns, but of course you have to be willing to wait out the siege or wait until a reinforcement army attacks you. But probably the best use for this army is to strike deep into the enemy territory and pounce on the half-stacks the AI is going to combine into a bigger force to send at you later. Bringing the fight onto the enemy's turf, even if you don't conquer their cities, can help take the pressure off your own cities -- which is good, because as Parthia city fighting isn't your strong point. The most casualties I get in my all-HA armies are due to friendly fire, and it's quite cheap, which is important for an early Parthia campaign. In my last game I felt like I was running around with a laser gun, just vaporizing whole Seleucid stacks with impunity. Great fun!