Your first objective is to destroy the ventilation shaft so that your troops can roll up inside. After you complete the objective, send one of your troops to the middle of the destroyed fans. Then, throw a grenade at them. Some will just duck and when the grenade explodes; they will fall down and die. To kill your own soldiers on this level, simply order one of them to go to the edge of the ice. Then either throw a grenade by him, or go over to him and punch him.
Eventually he will fall in the water and freeze to death. The best way may be to get a rocket launcher (enable the code if needed) as well as the "Flymo Ragdoll" code. Shoot the rocket near the man. He will fly in the air then fall into the water. It will take him a long time to get up, but when he does he automatically dies.

Go to the edge of the level and tell your people to defend. Step back and enable the "Rocket launcher" code. Shoot them and they will fall into the water and die.