World War Z

Most likely you will get this achievement without trying if you are going for all the others. But if you somehow don't you can easily get this on easier difficulties by picking up a RPG Heavy and waiting for them to charge. Once they charge blast him with the RPG and should be a one shot kill.

Additional things that may help - Running the Gunslinger class with the auto mark special zombies and 25% extra damage to special zombies perks.

Playing offline or with a group of friends may help with randoms not killing the bull before you can.

Can be done on the very first level, New York - Descent on easy, offline. Pick any class and just shoot the Bull when it is charging. The bull are the large zombies in the police Riot gear with the helmets. You need to shoot the helmet off first to bring it down. I got this on my pistol-only run and just spammed the trigger while aiming at its head when it charged. On easy they go down after around 5 shots to head, 4 to pop the helmet off and 1 to bring it down.