This will only count for "Infected". Walkers will reset the counter.
Gas Station south. There's a Tunnel where infected will continuously Spawn. since Infected dies in 1 hit you can wait for them to show up one by one and kill them for the achievement.
(If they somehow stop spawning, run twords the gas station and back to re-spawn them)
Note: Remember only kill Infected, hitting any other zombie will reset your kills for "One is all I need"
Also: One Punch man Reference :)