To beat the apocalypse incarnate, the Mongols, you need-
1. A strong state, you had to rush to compose this and keep it together. Put your funds to acquiring territory that can supply your war machine, buffer your important regions, and maybe a nest egg to hide in and cry in after all your shit is taken by horse riding devils.
2. Location, the farther away, the safer you are, because everyone in front of you has to get murdered for them to eat you. Buffer states are great, however they need to last, if not eat them yourself.
3. Alliances-Anyone who can send a good sized force is great help. Hopefully your brothers of faith intervene.
4.When in doubt attack first- the AI never breaks treaties, so attack them for a shitty duchy or something. You will lose, but you can thin them out and only lose what little cash you have and the little respect you have left.
5. Wait for them to kill each other, the hordes are greedy pigs and will murder anyone including the other horde, after the hordes clash ,if it happens, go nuts and eat them in a mad frenzy until the area of that map is your color.
6. Try to funnel them, hate that guy to the south of you? DOW the horde nearest and in the treaty time they will have already killed him for you.