Sassanids have to be the easiest faction I've played so far.

1. Recruit Clibinarii
2. Flank Romans while shooting them with arrows
3. Charge Romans from multiple angles
4. Get coffe, come back 5 minutes later when their entire army is dead and you've lost 3 Clibinarii, which is apparently Latin for "impossibly difficult to kill."

Sieges can be a little rough against Comitatenses in the early/mid years but you essentially can not lose battles in the open field. You can have armies of peasants, but as long as they're backed by Clibinarii or Immortals all they have to do is hold the melee line long enough for your HEAVY cavalry to waddle around the flanks. Opposing cavalry functionally doesn't matter since it'll bounce off the Clibwall.

Did I mention Clibinarii are really good? Best record was a C and CI (yes, that's 2 units, alone) versus about 700 roman infantry and light cavalry. Persia represent.