An interesting option. It makes things a little wavy, adds a bluish tint, and makes you fall slower. It is like you are under water. Your not supposed to get hurt that badly anyway. It can only be used by Marine and Predator, and only on levels with Aliens.
Aliens tend to blow up a lot when killed, and their bodies melt away in seconds.
There are only 17 aliens in the entire level, so your permitted margin of error is next to nothing. Take them on one at a time, shooting them at their feet with you pistol so as not to blow them up. Save your game every four or five successful trophies. Above all, be very patient. Do not use your shoulder cannon.