Copying straight from the KF Wiki,
"The DOOMed map is great for powerlevelling Berserker on solo, as it has a chainsaw spawn very close to the player spawn. High difficulty setting combined with the Faked Players mod can yield hundreds of thousands of damage points in a very short space of time."
I used this to powerlevel my zerk from lvl 4 to 6 combined with playing in matches to get a better feel for zerk tactics and skills.
What I did was set the FakedPlayers mod up to like 20 or something like (gave me about 300-400 zeds to kill on the first wave), grab the chainsaw in the back of the truck, and stay in the mouth of the truck and mow everything down with that chainsaw.
In the meantime, I also lvled up my demo from lvl 4 to 6 as well (your butt is still parked in that truck with the chainsaw spawn). Due to the really high faked players # you set it at, the game can only spawn a certain # of zeds on the screen. What happens a lot of times is that zeds you miss killing start clumping towards the middle of the parking lot where the truck is at. (AI pathing issue at play here) When the crowd in the middle grows to a sizeable amount of 20 zeds or so, I lob a nade on top of the group, and watch 15-25 zeds go boom. Due to the massive clumping, the zeds are not able to scatter normally and therefore are not able to execute that damnable troll nade-dodge manuver. I usually use up all 3 nades in my arsenal while mowing zeds down with the chainsaw. Good way to lvl up zerk and demo at the same time.
Once wave 1 ends, you have 2 options - either restart the map and redo wave 1 or give wave 2 a try and kite your *** off. (Staying in the truck won't work... you'll get sirened to death or fireballed to death.) With 400+ zeds, it becomes a real challenge surviving that many zeds at once without getting sandwiched. I don't think I've ever survived past wave 4 with 20 Fakedplayers activated.
Albeitly boring, once you get past wave 1, then it gets exciting. Hopefully this will help someone to powerlevel your zerk and/or demo. But like what others have mentioned, never rely on powerleveling as means to an end. Play with the perk in MP matches to understand it's role, strengths, weaknesses, and how specimens react to certain attacks in a team-setting. I might have powerleveled, but I was able to learn the tricks of the trade and successfully kite maps on Suicidal and HoE.