That first battle is such a mess to deal with. The only reason why you can't rush in right away is that the cannons screw it all up. You have to lure them out first, along with the Onagers, and then the battle becomes managable. Sub 8:00 IGT is definitely possible, but it depends almost solely on whether you can take out the enemy cannons quickly so that your trebuchet doesn't get wrecked. Most other units aren't too much of an issue.

I'll improve this IL in the near future, since I'm not too happy with it yet, but it's a start. I had a run that was on pace for being close to sub 8, but it died on the last cannon tower hitting my cart!