This map isn't so hard, compared to other OA maps and not as time-consuming. It can be a quick and dirty fight...

Your armies have retreated beyond the river. A last few units are still in the city. Immediately move them passed the bridge if you want them to survive. The enemy already has a few squads on the west-side of the river and will attack you, but you should be able to overcome this with minor losses.

After the attack, reinforcements will follow. Get back to the other side (east), occupy the town and then fight your way to the two enemy bases.

In the south, on 'your' side of the river, there's some abandoned artillery. You might want to use that to your advantage.