Hell Base Chapter 4 (After taking Relic)
As seen in highlighted clip providing by Iron Goat Gaming. This area is prime hunting for this achievement it gives you plenty of opportunity and great clustering.
Best to take care of the Snipers Rifting in on the Balcony first. This will allow you to take your time and build your shots as the enemy funnels in through the corridor.
I stood back by the Throne and zoomed with my scope to get proper alignment to set up my shots.
Hope this helps!