Meh. I've made it to 25 by myself. You're gonna need the Wunderwaffe at this point, and its strategy is nearly impossible to use now. They made it so there are narrow doorways, many more Zombies, and so the Zombies can spawn anywhere due to it being quite a large Map. The Radios give you the origins of the Zombies/Hellhounds, as well as info for the next Map. Area 51. Calender has every Thursday in September circled. The 24th is cut off, as well as 7 weeks after the release of Map Pack 3. Apparently the 7 weeks bit is important, but I'm clueless as to why.
There is a hanging man across the street from Teleporter 2/B. There is also strange writing on the walls in the same building. If you walk over to the boiler with a body in it, find the ladder, and go prone, hold X to hear the Zombies invading Der Riese, as well as the man committing suicide. Pretty creepy.
Haza, the Bowie Knife is on the outside wall of Teleporter 3/C. The one just behind the Power Room. Quite useful when reloading on a Dog Round.
The Monkey Bomb is generally more common than the Ray Gun/Wunderwaffe. Odd that you haven't found it yet. Unfortunately, the dogs are not influenced by an annoying Monkey. Gotta stick to fillin' 'em up with lead.
Even though other users suggest using a wall gun for a safe amount of ammo, I say this. Use an MG/Ray Gun. They have a gigantic supply of ammo, and are much stronger than other guns. The Gibs-O-Matic, Afterburner, and Spatz-447+ all run out of ammo very quickly. The upgraded MGs are much more efficient. If you want to replenish a Wunderwaffe/Ray Gun/MG or any gun you can't get off the wall, just throw it back into the Pack-A-Punch for more ammo. I suggest using a Monkey Bomb to cover yourself while doing this, though.