The best place I found for this was in Chapter 15. When you are going around the outside of the Planetarium, you can hit five with an Tommy or Grease gun as they come around to stop you.
There are two that rush you the first time if you're quick enough, you can get those two and the guy behind the pillar on the right, the shotgunner next and then there's two more that charge you with a Tommy Gun that make up 6 enemies.