For this one, you need to earn 300 000 points on any arena of the Slaughter mode.
Few tips :
- go to harbor arena
- use the military truck
- NEVER drive into big fat zombies (green ones), instead try to drive as close as you can to them, and as fast as you can. This will made them explode and take some of their mates to hell with them..
- pick up health as soon as it pops on your screen, because they don't stay here forever
- Try to keep the speed up, if you stop for too long, you're dead
You should be good to go ! (I did 624 500 (20 waves) on my 3rd try..)