Fighting the Byzanites with early units is tough, at best. The problem is, their Byzantine infantry units go through anything like a shark though a surfer, even cavalry, normally the bane of sword units. The Varangians are even worse. A good idea, if you can´t kill the general fast, try to kill as many of the Byzantine infantry and Varangians with your archers, every single one counts here. From High on, things go way smoother, but that´s still 31 years away for you.