Heres what I did on 'The Mall' and got around about 17k & I'm currently 882 in the world on that particular map. The map is entirely full of the fully mutated creepy things that look like beserkers from Gears 1.
Anyway, Those guys have no tactics whatsoever and will just run straight at you. Where you start there is a sh*t ton of flammable tanks, use these to get you going with the first few.
Then when you enter the room that you have to slide under 'cos of the bust door, wait until they have all come out, there should be about 10 of them, drink the bottle from the table (adds intoxicated to all kills) stand in the corner of the room & wait for them all to pile in on you, then using your peacemaker (standard machine gun) use the charge shot and shoot at sea of bodies, the bullet will penetrate most of them & you get the x-ray skillshot, use up all 3 if necessary to kill them all off. At this point I had about 8/9k points which puts you in good stead for the rest of the mission, be sure to change weapons at the dropkit when you get out of this room so you can get a better variety of skillshots.
The mission ends with you kicking the ubermutant guy into the propellers of a helicopter (1250xp) this aswell as a decent time bonus should get you this achievement nice and easy.
This is my first solution so go easy on me guys.