Win condition: Defeat commander
Keep your forces bunched together and head toward the right side of the map, just outside of enemy range. Send Alm and Lukas (or two strong units) to deal with the first two brigands. From there, let your army catch up to take out the third, or simply finish him off.
When you can, eliminate the archer using two strong units. Next, take the Brigand commander head-on. He deals more damage than the other brigands, but can't one-shot any of your team members. To reap the most XP possible, defeat all brigands before taking down the boss. Your reward for defeating him is Ram Wine.