Hey guys, thought I'd post my way to get to level 20 without the need of perks etc.
Requires 4 people who preferably are your friends or have headsets and will do it with you.
Waves 1-2: Get as many points as you can. 4 shots with a pistol and then knife them is my way but you can do whatever.
Wave 3: Open the door on the left towards the Double Barrel. Open the door after that and if you have money left buy a Gewehr-43 or just save up.
Wave 4-6/7: Open up the stairs and head to power and fend off and you should have around 2-3k each by wave 6-7.
End of Wave: Keep one zombie alive and open up as many doors as you can and link all teleports. VERY IMPORTANT: DO NOT open the stairs that are near Trench gun and Juggernaut.
Wave 8: Here is where the plan really kicks in. All four people head up to Juggernaut and two people take a window each whilst two people take the bridge. Decent weapons aren't required just yet but they will be later.
Wave 9/10: Once you get enough money you should start using the chest or, and I would advise, getting decent weapons that are on the map. It is great if the person on the bridge can get an MG42, Wunderwaffe D2, Ray Gun or Browning M1919 but that is based on luck. The weapons I favor are the MP40/Thompson ( Both awesome when upgraded ) and a Trench Gun. ( Also awesome when upgraded ) If you can get a Ray Gun and a Browning M1919, then rock on.. you're set.
Wave 11+: Same as above, but instead of buying weapons, you purchase upgrades. An advantage to have weapons weapons on the map is that the ammo is there as well. Keep a zombie at the window when purchasing upgrades at the end of rounds and buy betties for dog rounds, back up if you become overwhelmed or for wave 16+ when dogs start attacking with zombies. For the Frequent Flyer achievement, you can just teleport in between the rounds whilst a zombie is at a window, HOWEVER, if you do teleport in between the later rounds then dogs will spawn. Be careful.
Well there it it. It has worked miracles for me and my friends so I hope it will you!
If there is any way to improve this guide, or if I have made a mistake, please comment on that. Sorry in advance if this does not work for you or has been posted before.
Thanks.