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Here's an easy way to get the "My Clothes" achievement, which requires that you quickly kill a Covenant enemy after using a supercharged plasma burst from your pistol about ten times. Basically, I recommend that you equip a weapon alongside this pistol that is extremely powerful close-range, like a sniper rifle. Then, while you're venturing though New Mombasa as Rookie, wait till enemies get close enough and stun them with the plasma pistol overcharge. Immediately switch weapons instead of reloading and blind fire with your sniper rifle. This will usually kill the Brute in one single hit, even on higher difficulties. Or, if you're not sure, just weaken them first. Just repeat and you will have the achievement in no time.