Start on RAAM's Shadow: Chapter 1. Play through until you get the "They are inside out perimeter" warning. Kill the grubs coming out of the hole to the left, then kill two (and only two reavers). After that, another grub hole will open to the far right, destroying the barrier between the base and the street. Kill the grubs here and once the E-Hole closes, head to the Hammer of Dawn Command Station. Kill all of the smaller grubs first and then the Reavers. Killing all the Reavers will end the segment, even if there are smaller grubs left.
I find this method is slightly faster. You can get 8-10 kills per 3.5 minutes. The other option is toward the middle of Chapter 4. It's a long chapter and during my normal play through it took me about 25 minutes to get to the command post and use the hammer. You still have to rinse and repeat several times. However, if you need all 50 kills and you want to do them at once then that may be faster. I suspect most people will have even a few kills with the Hammer before they clean this up though.