I see that this question is quite old, but I'll still add a few words.
I'll only say about the tactics that we used in Kino Der Toten, since it's the only map where we got to level 40+.
The following guide is for 2 players:
- Each player takes 2 windows
- Let the zombies break through and knife them. Letting them in would grant you a few extra points for repairs and a chance to get some power up
- Repair the windows(barriers) and wait for round 2
- In round 2 keep on knifing zombies, but don't forget that now you need to knife them twice to kill them
- In round 3 shoot all 8 bullets of your M1911 in the body of a zombie, then finish it with a knife
- Approximately at the end of round 3 we had enough money to advance further
- You will need 1750 and 2000 points. The one with lowest score should open the door that is located upstairs(costs 750 points), the other one opens the next door immediately(1000 points)
- If you are lucky one of these 2 rooms will contain a Mystery box. If you feel lucky you may gamble and try to get something decent. However, you may wish to stay safe and purchase MP40 which is located downstairs of the last room you opened.
- Once you are there, stay there till both of you have 3750 points. One player takes the top floor(the door and 1 window) and the other player takes the bottom(2 windows). I'd recommend switching every round, because at the top you get a lot more zombies, therefore a lot more points.
- When both of you have 3750 points(a little less would be fine too), open the next 2 doors(each cost 1250). Then you do want to turn on the power and grab a Juggernog(remaining 2500 points)
- Another option(a much more risky one) would be to purchase the Bowie knife for 3000 points. In this case you will still be a 2-hit-target for zombies, BUT zombies will become a 1-hit-target for you until round 9. So if you can grab this knife in round 6 or 7, do it!
- Comparison of the Bowie knife to Juggernog(*)
- The next thing you want to do is to activate the Teleport. Once this is done, repair all barriers and finish off the remaining zombies.
- If you have a Bowie knife run around and kill the zombies. Each kill will bring you 130 points, so you need to knife 24 zombies to get your money back(24*130 = 3120). Don't forget to get the Juggernog ASAP.
- By the second dog round(normally round 10, you can still 1-hit-kill them with a Bowie knife up to the third dog round) you should have:
- After that rush to the Teleport and wait there for the next round to start. Try to kill as many zombies as you can, but don't wait for too long, you will be overwhelmed and eventually die. So when you get 1 or 2 hits, use the Teleport
- Once you are up there, throw grenades to the theater, buy some more from the wall and throw again.
- If you have 5000 points and a weapon (very) low on ammo, Pack-a-punch it
- When you are being teleported back, you may appear in a room that may contain some power up. If it's there, take it!
- When back at the lobby, kill most of the zombies, but not all. Also beware, because zombies may be awaiting you from behind. LEAVE A FEW ZOMBIES ALIVE, MAKE CRAWLERS IF YOU CAN.
- With a few crawlers you now have enough time to run around, repair all barriers, visit a Mystery box a few times or drink some perks. And the most important: LET THE TELEPORTER COOL DOWN.
- Initiate link to pad, activate it, finish off the last zombies and repeat steps 12-17 as long as you can
- Single hit zombie kills up to round 9
- No ammo/reload needed
- Crawler zombies won't release the gas cloud if you kill them with a knife
- Costs 500 points more
- You are still an easy kill for zombies
- You will have to buy the Juggernog anyway
Weapons that you can get from Mystery box and do want to keep them:
- AUG: Decent damage, lots of ammo(30+270), fast rate of fire
- Commando: Good damage, lots of ammo(30+270), decent rate of fire
- Galil: Good damage, tons of ammo(35+315), decent rate of fire
- RPK: Decent damage, tons of ammo(100+400), decent rate of fire
- HK21: Good damage, tons of ammo(125+500), fairly slow rate of fire
- Ray Gun: Awesome damage, some ammo(20+180), slow rate of fire, wonder weapon
- Thundergun: Instant-kill damage, very low ammo(2+12), slow rate of fire, wonder weapon
Upgraded version of these weapons will feature more damage(except for the Thundergun), more ammo and some minor upgrades(such as scopes, dual mags for Commando, increased mag size, etc.).
By following this guide we managed to get to level 40-something. Hope it will help somebody some day :)