The best map to do this on is Roadhouse. To find the weapon you need, first go outside. Look into the far corner. There will be a door with the purple health thing to the right of it. Go through the door and into the room. Ahead of you will be a portal. Go through that. Then walk around the top, until you get onto a bigger area where doors are to your sides. Back up alittle and there will be alittle ledge to your left. On the ledge will be this gun.