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.