Anusyia and Arštibara
Don't hate me for this, Rez
Well, first we should explain what these units were.
The Anusyia were the "Immortals", though that's a name Greek writers mixed with the Persian word for "Companions", so they most likely were the "Companions", not the "Immortals".
There were always 10000 of them, and they were the main field army. They didn't have to be nobles
per se .
The Arstibara were the Palace Guard, so to speak. They were drawn from nobles only and, from what I remember from Rez' lessons, serving as a palace guard was basically their military training.
A historical (and effective) Persian army would be lots of archers (Thanvabara), who are deployed behind a shieldwall of Sparabara with supporting Takabara and local AOR units + 1 or 2 units of Anusyia in the middle of the line, serving as both Sparabara and Thanvabara. Then there was cavalry on the flanks.
Try it, it is both good as an offensive tactic (move in range, pepper them with arrows and kill the remainder with your 1337 Takabara and Anusyia =P), or as a defensive tactic (stay put, pepper them with arrows and kill the remainder with your 1337 Takabara and Anusyia =P).