If you are fighting in the high period then dismounted chivalric knights make short work of everything the byzantines have to offer. Easy way to get some is just build royal knights (and dismount them). Its a small unit, but can chew through katanks fairly easily, and also defeats VG and byz inf. One tactic I do is build about 6 units of these guys, dismount them and send them in first against any Jedi Byzantine Generals. They will be defeated, but will inflict heavy casualties on the byzantines, and also stand a chance of killing the commander, as when faced with a small army the Ai tends to go stupid and send their cav way out in front with no back up. In early period, well its much harder, but massed archery focus firing on the General is probably the best bet. Javelins are also good (if you have Spanish Jinettes, use them. I often bribe El Cid as soon as i can, no matter which faction I am playing, just to have a province that can build Spanish Jinettes and also gain a cool general).