It is true that these missions are difficult, but not impossible.
I failed several times, before I started to understand better how to attack the thieves' hideout.
1) Use the environment to hide and protect your soldiers. (Hidden behind a hill, behind a thief's hut)
2) Approach (without your men) a spot with the fewest bandits (alone or on patrol at the start of the mission)
3) Shoot a single arrow at a bandit. Wear your shield and walk and come back towards your men, but the shield facing the bandit.
4) You are going to have the advantage. Your men are safe from arrows. Once you have sufficiently attracted the bandit(s) to your men. Launch the charge (F3) on the bandits.
5) Important, once the batch of bandits is dead call your men back to you. And repeat the procedure for each enemy spot.
You will suffer a lot of arrows. If your shield is badly damaged, take a new one from a dead soldier or bandit.
