From Halopedia, the Halo wiki
The achievement does not require that all of Blue Team never die; at least one member never dying is enough to grant it. The achievement will not be awarded until the level is fully completed, including the cutscenes, meaning it will be granted at the start of Guardians.
While possible on solo, it is best done on co-op for communication with one's teammates. Should this achievement be done on co-op, it is recommended that Frederic-104 be selected due to his loadout containing a DMR and magnum along with 3 frag grenades. The extra frag grenade can be useful in taking out groups of Promethean Crawlers throughout the level, while the DMR and magnum allow Fred to score up to 81 headshot kills before needing to replace his weapons, assuming he hasn't refilled ammo for either of them. Although Kelly-087 and Linda-058 have the Shotgun and Sniper Rifle, respectively, they are slightly less useful as the Shotgun is only good for taking down Soldiers at close range and the Sniper Rifle easily runs out of ammo, even more so than the Shotgun. It is also recommended to have few or no skulls on to make the mission and achievement easier.
Should three of the four characters of Blue Team be downed, the surviving character must quickly fall back to a safe place to allow the other players/members to respawn as quickly as possible. The enemies to watch out for are Soldier Captains with Splinter Turrets, Armiger Snipers with Binary Rifles, and Promethean Knights (especially Knight Commanders) with Incineration Cannons, as the latter two enemies can instantly kill a character and make him/her unrevivable until next respawn, while Soldier Captains cannot instantly kill a character and make him/her unrevivable but are otherwise extremely dangerous with their Splinter Turrets.
Upon reaching the triple Warden Eternal boss fight at the end of the level, it is actually possible to skip it by taking one of the man cannons in the arena (preferably the one on the right), boosting towards the platform that eventually leads to Cortana, and then doing a ground pound onto the platform, clambering onto it, and simply running to the room at the end. This still unlocks the achievement, so long as at least one player stays alive throughout the entire level.
The following criteria will block this achievement from being unlocked:
- Dying on solo and being sent back to the previous checkpoint (this can be averted by simply returning to the main menu as soon as the player dies)
- With 2 or 3 players, all 2 or 3 players dying (regardless of whether or not the remaining NPC teammates are still alive)
- Manually reverting to last checkpoint (regardless of whether or not the Iron skull is on)
- Using Score Attack (this disables both of the mission's cutscenes)