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):
- Use the M4 instead of the shotgun and go exclusively for head shots (still use the pistol for the first two levels). This helps you rack up some early points and gives you a head start.
- This is kind of a given but get to the box as soon as possible and start farming for heavy machine guns, the plasma pistol, or the specialty weapon of the level (IE: the frost gun on "Five").
- Get in the habit of leaving a "crawler" or zombie without legs at the end of every round, by round 15 this is a MUST since you will start completely running out of ammo at the end of every round. You can create crawlers with grenades or by shooting the ground with a "pack-a-punched" laser pistol.
- Perks are your best friend. I always go for Juggernog first, then quick revive. The other perks are great but I find that I usually die while saving someone else and these two help mitigate that problem.
- PACK-A-PUNCH! Each level has a pack-a-punch machine that costs 5000 to upgrade your weapon. After level twenty the zombies are so strong that pack-a-punch is extremely important.
- Communicate. Communication is key in this game type, when things start getting out of control it is easy for everyone to start running and get themselves cornered. If you find your window/door/hallway is getting overrun, let your team know! Don't wait until they see your revive symbol to tell them there are zombies behind them! You don't always have to stay together since splitting the horde can be useful on certain levels, but you should at least know where and what your teammates are doing. A good exit strategy and secondary meet up location are also useful if you do need to cut and run. Keep in mind some of the higher rounds can last 30+ minutes so moving to get more ammo may have to happen.
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.
