Exterminator

Exterminator is an achievement in Halo 3 and Halo: The Master Chief Collection. It is unlocked by beating the par score on the level Floodgate. The achievement is worth 10 Gamerscore. It is represented by a blue circle with a horned medieval helmet, also known as the "Warrior" emblem, in the original game and a screenshot of Ghost Town in the collection.

The par score for Floodgate is 15,000 points in Halo 3 and 25,000 points in The Master Chief Collection; the difference is due to the fact that the two games feature different campaign scoring formulae. Though the achievement can theoretically be unlocked on any difficulty, more points are awarded in campaign scoring when playing on higher difficulties.

In the original game, players must achieve the par score individually, even when playing cooperatively; in The Master Chief Collection cooperative players share a single score, potentially making the achievement much easier.

Tips
The Catch Skull has no effect on the Flood as they don't throw grenades. Because this skull will not make the Flood any more dangerous, it is an excellent way to add a multiplier without making the level more difficult. That being said, Marines and Elites are affected by the Catch skull, thus the player must be wary of being fragged by friendly fire.

Trivia

 * The level is aptly named, because it is the first level to feature the Flood in Halo 3, and it lets the player "exterminate" them.
 * In the original game, this is most likely one of the most difficult levels to score 15,000 points in, as it is the shortest level and the Arbiter and Elites can steal valuable points.