Guerilla is an achievement in Halo 3 and Halo: The Master Chief Collection. It is unlocked by beating the par score on the level Sierra 117. The achievement is worth 10 Gamerscore. It is represented by a blue circle with a gorilla face in it in the original game and a screenshot of Cold Storage in the collection.
The par score for Sierra 117 is 15,000 points in Halo 3 and 13,000 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.
Method 1 - (easiest) Turn on the Thunderstorm, Fog, Famine and Iron skulls on multiplayer co-op on Normal. You can use a "dummy account" to survive while you go out there and kill people. In multiplayer, the Iron skull will make you go back to your last checkpoint unlike in single player where you restart the whole level. You get a 3× multiplier even though it's almost playing normally in single player. It is important that your "dummy" stays alive and in a safe place.
Method 2 - Turn on the Thunderstorm and Fog skulls. Losing your motion tracker will not be a problem if you know the level well. The Thunderstorm skull won't make a noticeable shift in difficulty with the Brutes, but all of the grunts will be very good fighters, so be careful. Because they will be the rank of Ultra, they will shoot accurately, throw lots of grenades, and kamikaze a lot more. These two skulls should be all you need.
Method 3 - Turn on the Fog, Thunderstorm and Grunt Birthday Party skulls. Using the sniper rifle, headshot as many Brutes as possible. They give about 600-700 points on legendary.
Method 4 - Turn on the Tough Luck, Catch, Fog, Thunderstorm, Tilt, and Mythic skulls on normal difficulty. Use lots of grenades and BR headshots on grunts and jackal snipers to earn tons of style points. As long as you don't die more than a dozen times or so, you can get more than 15,000 regardless of time.