You have kerns, of course, I forgot you would. Well, a line of Halbs (be sure to have moral boosting buildings in the province where you turn them out) with a line of kerns behind should be effective against any armoured opponent. Once the VG/Kats are stationary on the points of halbs, let fly with the javelins. You'll take a few casualties from friendly fire but thats war for you. If you can get the kerns out on the flank and throwing into the rear of the Byz that's better. You will probably need to take them off skirmish to be able to use them.

Don't allow the Kerns to melee they will die like the peasants they are. Once they are out of javs either save them for chasing routers or just withdraw them.

If you play as the Byz you'll find you worry quite a lot about enemy cavalry as said above so be sure to pack the best knights you can too.