First you need to pick ,,First Blood" level. Then you look at the weapon that guy next doors wields. If its a shotgun or Double barrel, rewind until its AK. Then you rush in, take the ak and kill everybody on the first floor. After you do it quickly before the combo runs out run to the second floor. (Then the combo is saved). Now you will have something like 15x combo just kill the rest of them.

WARNING!
This achivement often glitches and you have to do it twice or even three times. Dont be mad when first time it doesnt pop up.