** Sputnik Skull Recommended **
At the beginning of the level when you exit the first area to the outdoors, turn right to see a large floating platform separated by a large gap from the other section of the level. Use a frag grenade to "grenade jump" across the gap and land on this platform. It may take a few tries, so ensure that you have a checkpoint to revert to that is close by. Once on it, head to the right inside the dark room. You will need to stare at the corner of room, turn on your flashlight and zoom-in using your sniper rifle to display "HI BEN" written in the corner.
You can also grenade jump from a different platform that is a little closer. To do this jump, you will want to use the same method, but do the grenade jump from the platform where you kill the two large hunters.