I got this achievement by playing The Sacrifice. My father discovered this trick and by using it we got between 2k-6k zombie kills per hour.
1) Load The Sacrifice Chapter 3 on easy or normal in single player
2) Play normally. Activate the generators and kill the tanks. Take an M16, Pipe Bomb, another pistol and a Med-Kit. Scout the area for more Pipe Bombs, Med-kits, Pills, gas cans and ammo. There should be ammo and Med-kits near the generator in the courtyard.
3) When the bridge lowers DO NOT RAISE IT! Raising it will cause tanks to spawn and your fun is over. Crouch down with the M16 and kill the never-ending hordes of standard infected that swarm you. If you placed gas cans, ignite them to save ammo. Special Infected will spawn and try to kill you so keep an eye out for them.
4) When getting ammo, use the duel pistols and watch for the Specials (you can stun unters in mid-pounce with a well timed melee) they will tongue/pounce you if they can and the AI on the bridge are reluctant to leave to help. Boomers aren't an issue if you can kill them fast enough. If there are more Pie Bombs nearby, use the one you've got to make this step easier. After restocking run back to the bridge and repeat.
Tips: Only heal if your health is yellow.
Pain pills when your heath is red, the AI will heal you
Burst-fire the M16 to save ammo and be the most accurate.
The AI will eventually run out of primary and be forced to use their pistols, more kills for you as the range is shorter.
I played Zoey as Bill and Louis are Special killing machines.
I hope the guide helps.