Dead Alliance

As you accumulate enemy (i.e. bot/human, not zombie) kills, you will build what are essentially scorestreaks, much like Call of Duty.

This particular one can be found on the Light Class 1 (with the crossbow), if you're just starting out like I am. This particular skill is defaulted to on the d-pad.

I believe it takes ~3 kills with no deaths for this one to become available - quite easy against bots. It seemed to be a little funky on tracking sometimes, but that might just be me not understanding the mechanics. Regardless, call it in 10 times, and the achievement is yours.