Buying guns help a lot. There's a tree house you can see after you open the door. With 2300$ shotgun you can reach wave 20 with ease.
it's easy mate, get a 350$ shotgun in 2 wave, get a 650$ pistol on 4 wave and open the right door when you have enough money.
and keep get money to buy shotguns ( O P )
cheapest shotgun is 1350$ Ronigsberg 543 which is pretty good
and sniper rifles are pretty good too. you can penetrate many zombies head with one shot
After the first door on space station, there is a shotgun on the wall priced at 1350, that can usually carry you to the gun vending machine and the end of the level. If your struggling to get to the point where you have enough the silenced pistol in the first room near the drugs will one-shot early wave enemies on headshots, just don't miss.
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.