Zombie Army Trilogy

EASY SOLUTION!

Just try to shot the zombies in the arm or leg, if it fell of, you got a +1
It comes naturally, bullets and explosives count both.
I got it after the first campaign.
Edit:
As i found out recently:
Headshots count as limb!

Quickest way to get this is to load up a solo match of horde mode on the map "dead end". Set it to cadet difficulty and 4 players.

When the game starts rush up to the ammo area and grab the preacher and all of it's ammo, then stay just outside of the ammo area.

The zombies will come from both ends whilst your up there so make sure you watch your back now and again.

You see see a big group of zombies together coming from the mist, you shoot in the middle of the group, sever most of their limbs, you turn around and do the exact same as before and repeat.

When you run out of ammo just simply pause the game and restart.

Btw explosives do not count towards the achievement