This can be easily gotten in the third floor Chapter 14, Vengeance, as the many enemies near the starting point are situated close together and are either dogs or mobsters with melee weapons. Enter the third floor with a melee weapon and kill the mobster in the room on the left, then the two dogs and mobster in the room on the right, then the two mobsters in the bathroom above, and then return to the room on the left and head upward gradually, killing each of the six mobsters that runs towards you. You should have killed 12 by the time you reach the thug and the mobster with the firearm in the windowed area at the end of the path. The Zack Mask will also be helpful although it is not necessary.
Head to Chapter 13, Assault. As police walk by the starting door, knock them down and steal their weapons, but make sure not to kill them. Throw their weapons outside to where the cars are, as the police cannot walk outside. Continue luring police to the door and throwing their weapons outside. Make sure that at least one baton is availabe for Jacket to use. Note that the game might slow down considerably as more and more police become weaponless. Once all the police have been stripped of weapons, run around with the baton and kill everyone. Getting a high combo should be very easy. Using the Brandon mask, the Graham mask or the Zack mask can be helpful here. While stripping the police of all their weapons can take time and be quite difficult, if one manages to pull it off a high combo is guaranteed.
Another, somewhat easier method would be to use the Zack mask on chapter nine, Crackdown. If all enemies on the first floor are killed quickly enough, a 12x combo will be reached, granting the achievement.