After following the RDA helicopter, you'll reach the Weeping Gorge. Ikran flight is disabled here, but you can climb to the top (where the video starts below) and pick off the RDA robots from a distance. When firing form a long way away some can get confused and makes it easy to slowly pick them off. Unless you have some sticky mines to throw at them.. this method works the best.
Listen to the Wind
Once you reach the summit, take a moment to listen to the wind. It is very simple to complete just make sure you aim your joysticks into the target area and do this until the end and you will complete the stage, and you're ready for the next step: the Call to Arms.
The first part of the Call to Arms mission is shown at the end of the video below. Don't worry, it's a brief introductory mission with a cutscene.
After this you can begin to prepare for the Call to Arms main mission.