This is essentially just a race to the finish sort of deal. As your allied officers engage in battle or your bases are raided they lose troops. The goal here is to win the battle before losing even 10% of those very troops. There are a couple steps to keep in mind while going for this.
Firstly, look for the shortest route to the main camp. The fewer bases you have to capture between your main camp and the enemies is less time spent, meaning less allies lost. Usually there is a shorter way, especially if you have the advantage in numbers. If you bring more soldiers with you in to battle than the enemy by a large amount, you'll have more bases from the start.
Another tip would be to make sure you have purchased the Red Hare in the town before attempting this, as it's the fastest horse helping you speed up your run. It will cost you some gold, but vastly help you in your cause.