Attack each with your best counter. A unit of 60 Swab Swords should be the equal of a unit of 100 Byz Infantry. Mounted Crossbows can take out Treb Archers if you flank them in open terrain. Pummel Kats with Slav Javs after you’ve pinned them with spears and make it a point to kill their generals. I believe that you can Crusade against the Byz, so you might hit them in Constantinople as Martok advises. The Byz are also on the tour list of the Golden Horde, so if you can hang on, they’ll eventually be in deep doodoo.