Gunscape

At the main menu press cn_RB once to go to Build. On the left side select Start Build. Then press down on the left stick to select Create New Map. Then Edit map to the right. Then cn_start to load the map. Once in the map press . Then select Test Map on the bottom right. Then Begin Test. The map will load up again, but in game mode rather than build. Press again and select Finish Testing on the bottom right. Then press again and select quit, then Save and Quit. Now go back to the build menu and you will see your map you were just in and now there is an option "Publish Map" on the bottom right. Select that and the achievement will unlock.