All I would like are some decent strategies for surviving zombies; I have gotten to 21, but I my goal is to live much longer than this.
Since you don't mention which level you are trying to last longer on it is hard to give you any specific strategies. However there are a few things I usually do (my highest level is 32):
There are plenty of other strategies but these are a few things I find help me survive a little longer. Playing with people you know helps immensely, I find that a lot of randoms would rather die with their points then spend a single one to help open a door or activate a trap. They also tend to try and do there own things as opposed to working as a team and alot of people are just out for the achievements. This isn't always the case, just what I have found the majority of the time when I play by myself or with one other person.