You can just walk out of the ditch and move into the field. The forces on the road can be ignored safely. Look at the map to make sure that you don't have any pursuers, then walk along the path to Dexter's position. There should just be one more group of 6 soldiers by the trees at the midway point. You should be more than ready to deal with them.
Run up to Dexter and realize that you're at the center of a rebel counterattack. Jump on the cannon and sight it for the top of the hill. It should be 2 bars up and dead center. Fire it as they cross to kill the bulk of the forces. When they're dead, turn it as far left as it can go to get a few shots on the next group of Confederates. When they're about finished, jump off of the cannon and rush to the back. There's a huge group moving in on the rear of the house. Stick to the sides and pop out to fire a few shots. Try to wear away at them for a bit, but keep an eye on the front. Another large group will rush the cannon position. Try to jump on and fire a shot into them before they get too close. There should just be one more group coming in to the left of the cannon.
Of course, it can go to complete chaos in a minute as soon as you go to the back. There are several cavalrymen, random infantry, and a few officers. If the frontal assault overwhelms the cannon, then just hang back inside the house and survive. Pop out, fire a few shots, then fall back behind cover. Just keep doing this until the rebels retreat.