On the part of the level were you have to defeat the enemies on the bridge, before that your see that, there is a path turning right.

Take that path and you'll see that there is a bridge above. You go to the end and turn right and you'll see some rocks, but dont climb up them.

Instead go on and follow the path and destroy all enemies in the camp, then there should be a chopper ride to the rocks.

Hit the rocks and you should be able to get up. Find the ramp and use boost to get over the ramp.

Then park the chopper with the front off the end, jump on the chopper and then jump onto the platform.