Complete Objectives 1 and 2. Use the Map Computer to find the river and the best path to it. Press Z two times to crawl and make your way to the river. When you reach near the river, you will be able to see the red line (not to be crossed) and "Danger" and "Mine" symbols. Do not cross the line. Walk besides it up to the end of the mission. Do not bother to kill enemies in order to save time and avoid tanks. You will be able to get near the helicopter. Cross the fence to complete the mission.