Caetrati are somewhat overpowered in 0.80 IMO.

use archers.
It sounds like he's playing with the Lusatonnan, wich don't have any archers. Balearic slingers are good though, if you can find them.

And/or use skirmishers with javelins to reduce their numbers. Caetrati don't have much armour fortunately. They have "very good stamina" though so it's unlikely that you'll be able to tire them out much this way.