Gravity GunG 50

In order to unlock this achievement you will need to finish the First Person Cloud level without picking up any ammo and completing the level objectives which are soaking 15 humans, destroying 15 towers, and beating the boss. I posted a link to a video of the best way to get through the level (in my opinion) and without any damage done. Just follow it and you'll get through no problem. Just remember when you get to the area right before the boss battle to use your snow on 1 or 2 additional humans to the 1 you already have. It isn't required but it will make the boss battle a lot easier and a lot faster. By throwing 3 human snowballs at him he should die in 1 or 2 hits. If you go in with 1 only, it will take a little more hits to kill him and you'll have to deal with the smaller enemies.