From Halopedia, the Halo wiki
Upon going deep into Apogee Station, reach the next wide area that spawns packs of Promethean Crawlers that are also accompanied by Promethean Soldiers. There are a few Gungooses available, so get on one of them. Make sure the other player gets on it too (either as driver or passenger).
The passenger should destroy the Crawlers that appear, using a combination of weapons and frag grenades. A Mongoose grants unlimited frag grenades to its passenger, so one need not worry about running out of frag grenades. The driver should use the Gungoose's grenade launchers to kill the Crawlers and Soldiers. The area spawns more than 40 enemies, the majority of which are Crawlers, which is more than enough to unlock the achievement.