I already know exactly what to do to build the plane, zombie shield, and Blundergat upgrade, but past level 20, when you just run trains or camp the whole time, I don't know what the best place to run trains is at.
And what is the best strategy if you run out of ammo for you Vritholic Withering, and other gun?
Camp at jug behind the big gate door, keep 2 of them closed, the jug door and the generator room door, there is only one door and one window to watch for easy 40.
Train at PaP, or build the shield in the cafe and train here setting the acid trap while shooting the blundergat at it, you shouldn't run out of ammo.
If you got a healthy score (100k+) and at the end of the round you are low on ammo keep hitting the box until the guard comes, after you kill him he will drop a random power up, repeat this process until you get max.
I found that once you get past 30, the best strategy is to train and turn on the trap in the cafeteria.
The fastest strategy is to camp until weapons become weak usually 40-, i dont know how you define best... best what? as triks stated you have to be comfortable, what may be best for you is not for others. Fastest training spot is cafe indeed and there is many ways to do it, i think setting it and then sitting in the little room with stears while it is in action would be the fastest in high rounds (50+), it involves specific doors closed, also depends if it is a solo or a team game.
The safest place to train is obviously bridge but you will get no drops there, roof is safe too but there is no traps here, showers a bit crampy but not too bad.
Best strategy for when you run out of ammo is to incorporate acquiring the Hells Redeemer into your set up. If I remember correctly, when charged at least once it's a one hit kill...forever? Exception being against Brutus, I believe. Though it has been a while since I've played.
The most popular location for running is the cafeteria. There is a few other places though, depending on personal preference.
Personally, I feel most comfortable on the roof, running around the plane. A friend of mine feels most comfortable in the basement running around the bodies. I'm not completely familiar with that strategy though. Could never get it.
Like Boybad said camping can certainly speed up the early rounds. The Cafeteria trap strategy is the best solo and co-op strategy for faster high rounds. You can make the rounds quicker and also get plenty of kills and build points by using the upgraded tomahawk while training but it is a bit riskier when zombies are constantly spawning. If you get into a good routine you can throw 3 or 4 tomahawks before the trap is ready to be used again.