This is cheating, but if you ever happen to be ambushed by a much larger party than yourself, you can simply press tab and retreat in the first 10 seconds of the battle to get of the ambush, however, some ambush sites provide a flat plain that is great for a tactic that can be described as 'herding'.
You will need a horse, and preferably a long spear...a sword will work if you're good enough. To herd the large group you will be facing, run through the blob coming at you, making sure to stay out of any spears that will stop your horse cold. After running through (and getting the entire group's attention) start circling around the group, picking off guys at the edge of the group. Eventually the group should coalesce into a densly packed 'herd'. At this point, circle closer and closer, keeping out of weapons range, and kill any foolish enough to drop their shield. Any guy that breaks off from the herd should be easily run down without support from his comrades.
This tactic works best against infantry with weak ranged capabilities like Dena raider or Franks (and non-cavalry, obviously). A flat plain is the best terrain for this tactic. Your weapon will break against groups larger than 20, so bring a spare. The best way to get the AI to lower their guard is to keep your shield up, and your weapon in 'neutral' position. It is very important to not get hit, one mistake can make killing those 20 Dena before an useless effort, keep your shield up unless you are using your weapon.
You might notice this happens anyway (the herding) when you are the only guy left on your 'team' and the AI chases you across the map in a large blob.
ALSO: Circling the group counter clockwise (meaning the group stays on your left side) will result in a more elongated 'orbit' around the blob, but will usually result in better charge bonuses. Circling the group clockwise (group stays on your right side) gives you a greater 'hit area', but you might not get as large a charge bonus as you normally get.
Personally I prefer the clockwise technique, as it gives you a larger area where you can hit.