A really easy way, i've found, to get the achievement "Tank Burger"(kill a tank using only melee weapons) is to go to Single player campaign and select No Mercy on Easy. Select the chapter Sewers and pick any character. Find a melee weapon and lead your team to the gas station. If you shoot the gas station with the other bots by it, it'll blow it up, and incapacitate or kill your team. Then try to get through the level and find a tank. If none show up, go back to the main menu and try again. Then, try to kill the tank with melee weapons. try to move in circles and back and forth so it may swing it's fist and miss. that's the perfect chance to swing your melee weapon. This may take a couple tries, but if it gets shot at all, you can't get the achievement. Another way may work with friends all picking up a melee weapon and surrounding it. a good place would probably be on Dead Center chapter 1, because then everyone could start with a melee weapon.