At the start of the level, turn to the right and immediately throw a grenade at the ground to do a grenade boost over the bench and onto the rock face. Once you are on the rock face, do another grenade jump to land on the higher rock. Walk on the top rock until you reach the edge, then jump and use the slant of the hill below you to slide jump to the left. Continue towards the flipped Warthog.
While walking towards the Warthog, you will pass a large green crate on your left. Once you get to the shadow that the crate forms, throw your last frag grenade towards the Warthog to flip it. The best way to aim the grenade is to aim your reticle so that it is just below the front right tire. The Warthog should flip to the left and land upright. Enter the Warthog and proceed through the mission.
Drive to the next area while avoiding enemy vehicles and stray brute shots. You will enter a tight area with a Ghost, a Chopper, and two Wraiths. Keep right at first to avoid crashing into the first Wraith and the Ghost. Typically the Chopper will hug the left wall, so most of the time it's better to stay right. The last enemy you'll encounter in this area is another Wraith, and there are two ways to get around it. The fastest way is to hug the right wall and hope the Wraith doesn't pin you against it. This saves about a second, but you probably won't make it unless you went really fast earlier in the level. The other option is to go around the left side of the Wraith, and there are almost no obstacles in taking this path. Once you pass the Wraith, you will find an empty Ghost. Exit your Warthog and immediately enter the Ghost.
Once you are in the Ghost, turn around and enter the tunnel. Boost through the tunnel, but stop boosting as you go over the peak of the hill. This will prevent you from crashing into the ceiling and losing speed. At the end of the tunnel, turn right and drive towards the steep rock face. To climb the rock face, boost up the the right side while holding the jump button to raise the nose of the Ghost, turn left to land on the flat side of the rock, then take a right and boost over the rock to land on the left path. Proceed through the mission.
The path will eventually open up into the area that is used in the Lost Platoon firefight map. Once you enter this area, turn right and boost over the rail just to the right of the pole farthest to the right. You'll land to the left of a rock structure, and you should head towards the the rock structure that is straight ahead. Once you reach the rock structure, drive towards the left side of the rocks, but to the right of the staircase. There will be a green bush on the ground right in front of the spot you want to drive up. Use the jump button to raise the nose of the Ghost and drive up the rocks. Drive to the right of the trees on top of the plateau, drive off the right side, keep to the left side of the Wraith, and head into the building that leads to the bridge.
Hug the right side of the building's interior and drive through the middle of the bridge until you reach the next part of the park. Turn right and drive up onto the cliff where the large bush is. Drive along the cliff until you can make it over onto the plateau where you would normally drive, all while turning around so that you are facing the correct direction to proceed through the mission. Hug the right wall until you see a Wraith. Once you reach the Wraith, drive to the left of the fallen gray debris on the ground and scale the rock to cut the corner. Drive along this path until you reach the final rock on the left side of the corner, then proceed to scale the left side of that rock. Drive down the rock and head towards the end of the mission until you reach a small hill. Hold the jump button so you don't flip while going over the hill, then turn right and drive through the wall. Getting out of your ghost while falling will make you trigger the ending cutscene faster.