Just go to Horde, any mode will do, beginner will make it a 15 seconds achievement.
If you start on Frenzy, you'll have 10,000 energy on your character.
Buy the Scorcher on the weapons section for 2,500 energy.
Kill any enemy, achievement unlocked.
UPDATE: It has been reported to been glitched, try restarting your game to refresh it, but there's a chance you need to keep killing with the weapon until it unlocks.