BC Romans can't stand a chance against horsemen factions.

I am playing Hayasdan (my favorite faction), and every time I am fighting the AS heavy infantry army and always being outnumbered, I fight in guerilla tactics, ambush, arrow shower, "bulldozer", hit and run. Results are multiple heroic victory.

So if you really want to fight the steppes or the tankers, I suggest deploy your army in defensive position first. Let your unit face the direction where the arrows are coming, less casualties. But watch out for cavalry charge. Recruit few long range archers to kill some unarmoured horse archers. Don't use your cavalry to fight the heavy cataphracts, only use them to flank light horses or chase when routing.

Deploy your army formation to Roman checker formation and not 3 lines formation. This you can trap those heavy cavalry charges without getting hit and run tactics from your enemies. Just like what happened in ZAMA.

I don't play Romans so I am not sure how long can the legionaries or the auxilia spearmen can hold cavalry charge. But a combination of Thureophoroi and Thorakitai can definitely kill even the heaviest tankers.