On Death Toll when you are at the train yard, if the tank comes climb the stairs to the top level.

Check every room - there could be zombies in and sometimes ammo packs.

Once you get to the top, the tank can not reach you but beware for flying concrete because it can knock you off the ladders allowing the tank to kill you.

Do this on single player, change the difficulty to expert and get an auto shotgun kill.

Your team mates reaload, get more ammo, change the difficulty to easy and go outside (it all depends on when the Developer puts the tank in the level).

Shoot the tank from the top story of the stairs and and kill the tank.