When you are rushing the hill on the easy difficulty setting, just rush up the right side. If you try to kill everyone first it will not work -- they just keep appearing. Note: The Vietcong appear on both sides after eliminating the first two turret bunkers -- rushing up the right side does not work on the normal or hard difficulty settings. Instead, stay prone and crawl up the middle. The turrets cannot see you, and neither can the Vietcong because they are being suppressed by the rest of your platoon.