Kill 128 enemies using secondary fire mode.

[ Register or Signin to view external links. ]

Secondary functions on the guns are activated by pressing RB, simply change to the secondary function of any of the guns you are using and start racking up the kills.

Also included in my solution for Deadly Laptop

In multiplayer, set-up a match where it is you against 4 peace sims and set all of the guns to laptop guns. The peace sims won't attack you unless you have a gun out and even then they just disarm you and run away.

Now you need to find an area with a laptop gun you can camp in. Just set a turret down and pick up/equip another laptop gun to lure the peace sims in. They will get mown down by your turret racking up the kills. When your turret runs out, set another one with your equipped gun and pick up/equip another.

Achievement is worth [ Register or Signin to view external links. ]