*Must be done on Normal or harder difficulty*
This is done on the mission called "Tip of the Spear". You can start at Rally Point Alpha. After you have cross the bridge with the Warthog, you will hear the AI say that there is a high value target is in the area. An "Elite Zealot". After you go up the stairs and come to a area that you see a turret at the end. Take a left, and this will put you inside of a very large open room. Stay there, and you will see the Elite Zealot come out with a Red Arrow above his head. Kill him quickly and this will be done.