Take a Hike
From Halopedia, the Halo wiki
After the Warden Eternal boss fight, the player must cross a series of chasms to reach the Gateway. Two Phaetons are available for this section, but none of the Spartans must ride them for the achievement. Instead, Blue Team must cross the chasms through a series of man cannons, battling Armiger Soldiers, Crawlers, a Promethean Knight with an Incineration Cannon, Focus Turrets, and Soldier Captains wielding Splinter Turrets.
This achievement can be done on solo or co-op. Either way, none of the members of Blue Team must be allowed to enter a Phaeton, including AI-controlled teammates, or else the achievement will not be unlocked.