It is earned when the player earns 15,000 or more points while playing the meta-game on Sierra 117. This Achievement gives 10 Gamerpoints to the player. The points can only be earned while campaign scoring has been turned on competitive or cooperative play
- 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 3x 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.
It is represented by a blue circle with a gorilla face in it. The actual term "guerrilla" comes from the Spanish word "guerra" and means "small war". Guerrilla is referred to in this instance by the UNSC Marines Corps fighting in Covenant occupied territory in the Republic of Kenya late in the Battle of Earth.
There may be an additional play on words: guerrilla sounds like "gorilla", a reference to the Jiralhanae, but the title most likely refers to MCPO John-117. This may also refer to guerrilla tactics because John-117 is moving in and out of the jungle using hit and run tactics.
- The software used by Bungie to edit the map tags is named Guerilla.
- The name is both a pun and a largely appropriate name for the Achievement: gorillas (the icon is a cartoon gorilla) have habitats in jungle areas, which is where the Sierra 117 takes place, and the fact that guerrilla hit and run tactics are largely prevalent throughout the level. Also, Brutes have a similar appearance to gorillas.