The trick I use when fighting them is a combination of horse + archers. I get them to chase after the horse, which will run up past my row of archers. The archers whittle down them nicely. Then, I either charge with the horse (if they've been taken down a lot), or circle around and repeat the procedure.

With a lot of enemy units, this can require quite a bit of micromanagement, but it can still be effective. Often, once the computer has 'locked' on to one of your units, they'll chase them all the way across the map before giving up. It's a good way to break up the army, and let you take on units individually.

Bh