Ok, the description of this one is 'Plasma Pistol Overcharge and quickly kill 10 Brutes'. I'm not sure if this is a mistake or a glitch but I got this easily by:

1.) Getting a plasma pistol
2.) Going up to a Jackal(The ones that sometimes carry glowing energy shields or snipers).
3.) Have the pistol charging(holding down fire, it must be charged for a short time to make this work right).
4.) Release shoot and quickly melee(The shot should stun the Jackal and the melee should kill him).

You don't have to kill 10 quickly in a row, you only have to do this to ten in the same level, I got this in firefight. Of course this is easier than doing the same thing to Brutes, good luck.