An easy way to do this achievement is play a job that requires camping, like loud bank jobs, cook off etc.
Sit on the objective and let the cops make a push, when you see a group of 2+, throw a molotov (throwable) at them and just stab away with the motherforker (melee).
I would recommend playing on overkill or above, as you don't want them dieing before you can start stabbing!