Kill the Elite Zealot before he escapes during the 5th mission. 25 points.
You'll encounter the Elite Zealot on Tip of the Spear shortly after Rally Point Alpha. You must kill this Elite before he escapes to earn this achievement. You can accomplish this with a Plasma Pistol and a Needle Rifle or DMR. Make sure you're playing on the Normal difficulty level or higher.
Fight your way across the bridge and around the side of the factory. Pick up a Plasma Pistol on your way, as well as a Needle Rifle dropped by a Skirmisher or Jackal. You'll eventually come to some stairs with a weapon rack at the top. The Elite Zealot will be in the large area beyond these stairs with a red arrow above his head. Hit the Elite Zealot with a charged Plasma Pistol shot to disable his shield, then equip your Needle Rifle or DMR and shoot the Zealot once in the head to put him down. This video is from our unofficial game guide. You can view the full guide here