While on the level, kill the first three little opponents. After that, two Lokis/Hellbringers and one Mad Cat/Timber Wolf will approach you. Take off one of the three mech's leg so it falls over and kill the other two. Now move directly opposite of the city so that you are heading to the mountains. There should be a couple passes that let you to the top. You are now at the edge of the map and can drop down. You will take some leg damage, but it is worth it. You cannot get back up, but you can now shoot your enemies from underground. They cannot hurt you, but you can hurt them.