This is a skill point.
Simple enough, at the start of this mission you want to go straight and down the path where the pine trees are in the snow. Once down there you will notice enemies that pop out of the ground, these are Anklebiters (they are clustered together). As for weapons to use, I recommend the Pyrocitor, Suck Cannon or R.Y.N.O (this will make it very easy). What you want to do is kill all of them, then wait for another wave of them and do the same. There is about 5 waves of them, so don't move on with the level until it says you have unlocked a skill point. The trophy will unlock shortly afterwards too.