Just play Britannia with their chariots. It is not easy to stop them. Here some tipps:

1. Use some range units, like bowmen to fight them from the distance. With Greece I faught Parthia this way. Lots of archers and javelins.
2. Try to stop them. Use deep infantry formation. Some cheap infantry can do the job - peasants or militia - just to absorb them. Have good spears behind. Then kill them with arrows, swords or cav.
3. Never try to fight them with your cav as long as they are mobile.

Good luck!