I got this achievement, 250 kills playing solo and in about 15 or so minutes without wasting my bullets. I started with only 23 kills from guns only. How?
Spoiler below ! :
Don't worry too much about the achievement from early on. When you reach Chapter 7 - this is after the supermarket level cut scene where Jin steals the truck filled with supplies and you have to head back down the tunnel and back to city hall where every one there turns into a zombie.
When you exit the sewers, you end up in a basement with the worktable.
Just follow the waypoint going up. You'll encounter about 5-6 zombies till you reach the...mounted machinegun! Tadaaa! From there you'll get a chance to take free shots of about 12-15 zombies with out wasting your own hard to find bullets.
After the zombies stop spawning just reload your checkpoint and you start at the basement with the worktable.
From here on you only encounter between 1-3 zombies randomly spawning and they are usually on their own.
Rinse and repeat, and achievement pops and bullets saved!
- Every reload of the checkpoint gives you about 12-15 zombies to shoot at with the machinegun.
- Do not shoot the barrels to your right and the truck to the left. The explosion kills the zombie and it won't count as a gun kill.
- If you count only 10 or so kills on a particular checkpoint reload, aim the machine gun to the upper corner. about 1 o' clock of the two explosive barrels is a wall, there are usually walkers behind that wall, shoot it even if you dont see anything. you'll be surprised at how many are grouped there occasionally.
-Make sure you close both doors before using the machine gun. We don't want to have any surprises now do we?
*edited for spelling.