Outcast - Second Contact

There are two keys needed to get the Mon in this location. The first is from the Organ puzzle, and the second is from the waterfall. You need to shoot the two smaller rocks to move the boulder into the water to stop the waterfall. Once you do that, you can go into the area behind it to grab the second key. Then make your way through the barracks, and at the top enter both keys to find the Mon. Hope the video helps!