Tying Up Loose Ends

Tying Up Loose Ends is an achievement in Halo: Combat Evolved Anniversary and Halo: The Master Chief Collection. It is awarded for killing every single Sangheili in the level Keyes, on Heroic difficulty or higher. In the original game it is represented by a white Sangheili skull over a purple octagon; in The Master Chief Collection it is represented by a screenshot of Chill Out.

The achievement is worth 10 Gamerscore.

Method
This achievement is a lot easier with Bandanna Skull activated, since the player will have an infinite amount of grenades and sniper rifle rounds. The player just needs to go through the level and make sure every Sangheili is dead by the end of the level:
 * 1) In the beginning of the level, opposite of the hole in the hull is a Covenant lance led by two Sangheili, spam grenades at them.
 * 2) After dropping into the coolant, there are three Sangheili fighting the Flood nearby.
 * 3) In the next coolant pool area, there are about two or three Sangheili fighting the Flood.
 * 4) Before getting to where the Mgalekgolo are, pick up the Sniper rifle and snipe the two Sangheili fighting near the Mgalekgolo.
 * 5) In the next area, there is one Sangheili manning a Shade turret. Snipe or throw a grenade at it.
 * 6) There are two or three Sangheili under the gravity lift. Occasionally, one or two of them will fall into the coolant below, so check again before leaving.
 * 7) After going through the cargo bay, the player will return to the corridors where the Covenant are fighting the Flood. There are two or three Sangheili leading the lances here.
 * 8) When entering the shuttle bay, there are three Sangheili on the second floor. One of them will usually be manning the Shade turret.
 * 9) Before going up to the third floor, the corridor leading to the first floor will spawn an infinite amount of Sangheili and Unggoy. Make sure the last Sangheili is dead before continuing.
 * 10) Once entering the shuttle bay's third floor, kill the Flood forms on the right, then look down and make sure no Sangheili is on the second floor extension.
 * 11) A Spirit dropship enters the shuttle bay and drops down a Covenant SpecOps team. There are one or two Sangheili here. While it is not required to kill them now, it is recommended.
 * 12) Outside the bridge, there are two or three Sangheili fighting the Flood. Kill them now, or they will be harder to kill once the Covenant SpecOps join the fight.
 * 13) After the cutscene in which John-117 kills the Proto-Gravemind, there are two or three Sangheili outside the bridge. Let the Flood take care of them.
 * 14) There are two Sangheili on the third floor once the player exit. They are backed up by Unggoy with Fuel Rod Guns, so be careful.
 * 15) Continue to the second floor, there are about two or three Sangheili on the way down.
 * 16) Once on the shuttle bay's second floor, while it's not required, snipe one or two of the Sangheili on the first floor. If they take cover behind the second floor extension's wall, throw a grenade down to force them out of cover.
 * 17) On the way down to the first floor, there are two non-SpecOps Sangheili in the corridors.
 * 18) On the first floor, there are two or three Sangheili inside the corridors. They are on the side where the Spirit landed. Throw a few grenades to weaken the shields, then go inside and take them out.
 * 19) Take out the two or three Sangheili near the Banshees, and let the last one be the one closest to the Banshee. Once the last one is dead, quickly board the Banshee. The achievement notification should appear the moment the cutscene started.