I've always had trouble fighting these. It seems like the only thing you can do is attack from the sides or back because even good heavy infantry get killed if you send them into the pikes or spears. They can turn around and fight when attacked from behind, so I'm afraid to charge cavalry at them even from the back. They hardly take any hits from arrows to the front. And the Selucids get these dirt cheap militia phalanxes that are so much better than my eastern infantry (as Armenia) and cheaper! Is there some downside to phalanx units?
Of course, I try to surround them with my cavalry archers so I can shoot their backs, but cavalry archers beat almost any AI-controlled unit anyway.