How do you deal with hordes?

Playing as the Franks, my Alamanni allies have conquered half of France and offer me a good stable east flank. But the HUns came, and the Vandals came, and the Suebians came.

The Huns - too many of them! The only way I managed to defeat them was in a clear siege battle where they had no chance against my spearmen - but in clear flatland, I stand no chance. Given that the Franks have few cavalry, and not top notch except those Frankish Lancers, any ideas how to deal with them? Spearmen and archers don't really work well when a unit of Nokkors destroys pretty easily what it charges into.

Suebians and Vandals are easier to deal with, but they're not helping either with their invasions.

Thoughts?