This is a three part mission - to go and protect three groups of soldiers until your ships can come and retrieve them. It doesn't matter what order you do the three groups in - all are off of a central stream area, in passageways marked by little glowing dots.
I first went striaght and jumped the stream to go up between the first glowing dots. There's a building and then the guys up on a hill. Your helpful voice points them out to you. This one is really easy. In essence you park the warthog on the hill in front of them and convince one or two guys to jump into it with you. You can easily blast anything coming up the hill before it gets to you. When you clear the bad guys out, your soldiers are lifted to safety.
Head back to the center area and go across the stream to area 2. This is the rocky one. I never find it worth it to drive the warthog into the rocks. Just park the warthog down by the broken ship and go in on foot. You can snipe the bad guys as try to get in to you. Again, once they're all dead, you're done and can head out.
The third area is the most interesting - there are several buildings. This again is easy especially if you're in two player mode. Just drive around the outside of the buildings and shoot anything that moves. They can hardly even touch you. When you get most of them, head into the buildings to get the last few that are down there. The door to get in is on the cliff side of the furthest building. That's it for this level.
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