It depends on the type of missile units they use. Slingers have a lot of ammo and annoyingly long range, archers are sort a middle ground thing and javelins have short range and no ammo but are a lot more dangerous than anything else. The good thing is that if you time it right, you can rout jav units with a unit of melee infantry by ordering them to charge and having the pilum volley wipe out half the unit. Only works twice though. Be happy you did not run into ranged cavalry yet. I have not yet, nor do I have an idea about how to beat them yet. I'm usually very heavily infantry focused, I don't like horses and fancy stuff so much. I guess I will try artillery.