This is an extremely easy mission. If you experienced difficulty in keeping your allied soldiers alive and found this mission very hard, then you probably made the classic mistake of trying to hide your soldiers in niches, expecting to take the enemy by surprise (thinking that if your soldiers were exposed, they would be shot down).
Nothing could be further from the truth. Your allied soldiers have long range rifles and will mow down anyone who approaches. Yes, even enemy snipers will not get a chance to target your soldiers. So, all you have to do is to lie down in plain sight of the entrances and turn on auto-fire ("Cover") mode.
When the game starts, quickly select your Sapper and place the anti-tank mine along the path to the main (middle) entrance, as shown in the screenshot. Run him just as quickly back to the courtyard. The battle is about to begin.
You have eight allied soldiers. Position four of them so that they cover the main entrance and two each to cover the side entrances (as shown in the screenshot). Make sure they are lying down and have auto-fire enabled. Use "Ctrl + Right-Click" to change the coverage area so that they have clear shots at the archways. This setup is enough to kill all enemy soldiers and snipers who try to infiltrate.
NOTE: For the main entrance, make sure that the coverage just extends to the entrance. If your allied soldiers are too far out in front, they will get shot by enemy snipers. As a guideline, refer to the screenshot. Your soldiers should be close to the statue in the square.
All you need to do now is take care of the tank and two armoured vehicles that will enter from the middle and the two side entrances respectively. Unfortunately, the anti-tank mine alone is not enough to stop the tank. You also need two shots from the bazooka for the tank, and one shot each for the armoured vehicles (which is just perfect, since your Sapper has exactly four rockets).
Lie down your Sapper at (A) along with the allied soldiers covering the main entrance. Turn on his bazooka and auto-fire mode (don't worry, the Sapper won't use the bazooka on soldiers). Sit back and wait. After a lot of enemies have perished at the gates, the tank will eventually appear, and get blown to bits. Then, move your Sapper over to the side entrance at (B). Once again, lie down, select the bazooka and auto-fire. Once the job here is done, move the Sapper to the position (C) and as before, wait in auto-fire mode for the second armoured vehicle.
Your Sniper has to do only one minor thing. An enemy sniper will lodge himself early on in the game near where the anti-tank mine is shown in the screenshot. He is the only fellow who doesn't attempt to storm the courtyard, so you have to shoot him down. Your Sniper can do this easily from the square.
That's all there is to it! Once all the stormtroopers are dead, the mission will be complete. You don't have to touch any of the enemies stationed outside. Easy, huh?
Addendum: We didn't use most of our resources, viz., molotov cocktails, anti-personnel mines and grenades. The Green Beret, Spy and Thief weren't used at all (and the Sniper barely so). You can do better. Especially, using the Sniper to pick off enemies is a real treat. As a bonus, if you put your Sniper to work on pre-emptively taking out enemy snipers, you can position your allied soldiers more aggressively (and don't have to worry about straying too far out into the open). The game is all about having fun and finding neat ways to accomplish missions. This mission is particularly suitable for experimentation, so explore and enjoy!