When you get off the boat, it is very tempting to race ahead and get past the gun turrets. However, you can hide behind the tripods and wait for them to reload. Then, make your way to the next one. You may be able to skip but it is not recommended. Then, just kill the Tediz's for the rest of the level. At the end you will find your commanding officer in an electric chair. When you get there, you can pull two different levers. One lets him go, and the other kills him.