Buy cath archers all the time until you get caths. Then if you want, you can replace some cath archers with caths, but it isn't even necessary in a long time. In fact you can do well with a combo army of 5 cath archers and 5-10 horse archers for a very long time if you just use clever tactics. You can even skip those footmen completely if you like to have more money for the cath archers. Armenia isn't a hard faction, they're as a matter of fact one of the easier once you find their strenghts. One VERY important strength is that a well-composed Armenian army only needs around 10 units to beat any other army in the game, whereas for example romans, barbarians, macs etc. need 20 units in order to have a strong army. If you bring 20 units to battle it's really overkill and the battles should be really easy to win.