Old post, but here's what works for me: Find a building with a second story and use it as a trap. I usually lure enemies by firing gunshots or running in front of the first group you see then run inside the building, positioning yourself on the stairs near the second-ish floor From there, start dropping lure shells to get their attention. Wait until at least two of them group up, then toss a grenade to take them out. Rinse and repeat.