There is a place in Oil Field Nightmare Arcade Level
where you can jump on a rock at the side of the map, i would recommend taking flare gun cross bow and a magnum with few flares. It is VERY easy to get on top of it and the taken cannot hit you.
--It may take a few tries before you get good at jumping on it, but its not hard at all--
Next step is to kill off the enemies until you see a taken witch throws those bombs [DO NOT KILL HIM] he usually stays in good distance - then just sit tight and wait for your achievement to pop.
What happens is all the enemies keep stacking up in front of you and the guy keeps killing them with bombs, note that you get streak bonus + points when he kills them and more he kills more you get!
Note that this will only work in Nightmare Arcade Level------->NEW VIDEO TO GUIDE COMING SOON IN FULL HD<-----.