Sauromatea Economy

[athanaric]

I doubt nomads can build mines...

All factions can build tier 1 mines. The ability to build them was added to the Nomad build tree in 1.1 or 1.2. Seemingly the team had found supporting evidence (thank God, it's already hard enough playing them).
Gava Saka doesn't have silver or gold though, which means you can't build mines there. It has only copper mines which add to trade income. The starting province of the Saka Rauka has one mining site where you can construct a mine.
Also, Sauromatae have the weakest economy in the game, and are victim to certain unwanted side effects of the recruitment system.

Regarding counter units, the locally available Dahae skirmishers are decent and fast anti-armour units. You should use them in conjunction with heavy cavalry (such as Aorsi or Roxolani Nobles) to counter enemy heavies. They're vulnerable to arrows though.
If you want to field infantry, try using the Slavic spearmen who're better than your native ones (fear effect and so on). Skythian Axemen are also useful vs armoured units. If you manage to somehow get regional barracks in Bastarnolandam or on the Crimean peninsula, you can hire pretty powerful Greek, Dacian, and Germanic infantry. However this requires a foreign, non-Nomadic government to be present in the settlement.