Same problem here. I did stages prologue to 5 twice, on hard, stage 6 one time, plus stage 3 three times on easy, and stage 6 one time on rank mode.
Doing a count of zombies of stage 3 on easy i get 625, x5=3125.
On seventh run throught stage 3 achievement pops up around 20th zombie. Now i have 2 theories, it's a 5000 kills achievement, or only counts zombies that you kill with the chainsaw.
If 5000 then 625*6=3750 + zombies killed in 2 playthrougs without stage 3.
If reaping-machine and expansive wave of explosions doesn't count then 225*6=1350 + same zombies as before.
I believe that the 2nd it's the right one.
Last edited by Toshin; 03-31-2013 at 01:24 PM.