The trick is to keep Edelweiss safe. To my knowledge, it is the only game over condition (or the number of turns).
You can keep it safe (behind a wall) near the starting point of the game when it's health bar is too low. Batomy will move forward at some point in time and leave it alone. Selvaria will stick near the tank but won't hunt for Edelweiss even if it's very close to her, as long as it's out of sight.
Someone else left Edelweiss parked near the bottom right of the corner.
The rest is up to your strategy. Most people got rid of the Imperial soldiers. Some left the turrets in to waste the enemy's command points. Whatever you do, just stay away from Selvaria.
I use Scouts and Lancers and focus on the enemy tank exclusively. I scored very low too