The achievement title is a little misleading. You can 'knock out' or kill any enemy in order to unlock this achievement.
By far the easiest way is to wait for the zombie outbreak after 2 residents die on a single estate. This will be slobs if you set them to produce workers and tenants from the outset.
Find your nearest build crew and use them to target zombies. You will have to micro manage your builders. selecting each one to attack a different zombie. You know when the zombie has been killed as a coin will pop over his head once it is dead. Repeat this for 4 more zombie and the achievement will pop.