Go to the start menu and select 'enter cheats' then enter these cheats:
Autowin - B, B, X, Y
Superman - B, B, X, A
Team Superman - B, B, X, B
Refills used (can be entered multiple times) - B, B, X, X
Purple Heart - Get wounded or killed in action
Bronze Star - Get 15 kills
Silver Star - Get 20 kills
Distinguished Service Cross - Get 25 kills
Congressional Medal Of Honor - Get 30 kills

Select the "Enter Cheats" option at the main menu, then press B(2), X, A. Your first soldier will be invincible, but the remaining members of your platoon can still be harmed.

Have a rifleman, a demo or support, and a sniper.Use your sniper when the threat indicator indicates that there are men from afar (yellow). Use your rifleman when it turns orange, and your support or demo when it turns red.