Well you kinda have to open the doors becuz you need stronger firepower. Umm, the best thing I can tell you to do is always keep moving from side to side and your best friends is either the shotgun (single and double barrel), ray gun, .357 magnum or the thompson. If you play with multiple partners it will be a little easier.
Once you get up stairs to the top floor. Make sure that one person gaurds their appropriate position. If they are any good, then you should have no problem getting to atleast level 25.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.