a helping hand - 5GP: swap an allies weapon for a Rocket Launcher, Fuel Rod Gun, Sniper Rifle, Beam Rifle or Brute Shot
skewer stopper - 10GP: end a players sword spree
pest control - 5GP: kill 5 drones within 10 seconds of each other
[Edited on 02.06.2012 3:10 PM PST]
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Hall Monitor - 5G - Surprise and make more than 7 Grunts panic. Graveyard Shift - 10G - Kill the marines on Sacred Icon. Gone - 20G - Complete any level by covertly killing Covies, either by sniper headshots, grenade sticks, or assassination. Anyone get the references? :3