How to fight Horse Arcehrs?

[ARCHIPPOS]

Also, @ ARCHIPPOS: Are Seleucid Heitaroi faster than Seleucid Katas?

I dunno abt that but for the upkeep of one cataphract you could be able to recruit 2 dahae (not as grandiose i know but very effective hehe!!!) ... i tend to use 2-3 FM members together (early Baktrian bodyguards which are inferior to the Seleucid ones ) instead of catas and it works fine (not to mention they get chevroned and gain ancillaries and cool warlike traits pretty fast) .

in fact i have been able to devastate Parthian and Saka halfstacks with 3 family members and 3 Dahae riders...(with minimal casualties ranging from 0% to 5% at each battle playing with medium or hard battle difficulty) ...

btw have you checked if you can recruit MERCENARY Dahae riders ???Perhaps you can invade Parthian territories and get a unit or two there (they'll take very little casualties anyway so you'll use them for quite long-but they'll cost you a small fortune) ...

also when taking a ridge with nomad enemies standing close occupy with Hetairoi ---> send the phallanx --->finally deploy the missile troops and start shooting ...
Don't use archers on armored foes (you'll just waste ammo-turn their "fire-on-will" thing off) DO use the slingers first instead...
I said earlier that you should NOT frontally charge enemies on elevated terrain... disregard that for your catas and hetairoi (they can do it taking minimal losses) .ALWAYS aim to kill the enemy general on the initial stages (using your Hetairoi)as this will make routing the enemy easier...