For space level I advise bizon 2k thousand dollar or something smg. You can purchase the auto pistol on the window at the start for an easy kills. Enemies have small hp and very fast where smg shoot fast and have a lot of ammo in a mag but deal less than assault rifles etc. Smgs are ideal for this level. You should either go to far left or far right (leave a little space to run back for reloading) to secure your back. Shoot fliying choompas first if you can or try to dodge(which is hard to time). Spider mothers are slow but will lunge on to you if you get too close. One last thing, if you can facehugged by those pesky little things you can get rid of them with the pipe that is flaming over your head by jumping(there's an achivement for it). I think you can manage level 20 with a solid smg. Good luck mate.