Simple! Use a combination of well armoured spearmen (with good shields) or heavy infantry (with good shields) and light cavalry (such as Komatai Hippeis). The strategy is simple, keep your cavalry out of the way and let the horse archers waste their missiles on your heavily armoured infantry. Then when they run out they should charge. Send your light cavalry nearby and at impact with your infantry they should all quickly die and run for their lives. They should at this point be very tired so your light cavalry can immediately chase after them and kill the rest.

If you can't find any well armoured infantry (I don't think the Dacians have much better than the Getikoi Stratiotai which are only lightly armored) then hire mercenaries.