Ah, the classic Ambush. :P
In my opinion it's a rather dodgy mission; sometimes your squad gets slaughtered, but sometimes you're able to complete the mission without any significant problems (especially in Cadet mode). Try to move with your squad members (long live AI!), be accurate when shooting and if possible, obtain a RPG launcher from a fallen hostile soldier. It'll come in handy later when you're moving towards the town (based on my personal experience).
If most of your squad is still alive after capturing the little farm west of the town, you have good chances to complete the mission. However, if most of your buddies are down, you're going to need a lot of luck - the town is full of hostiles as you surely know, and the situation gets worse later...
Good luck with OPF! :)