First, a lot of people (myself included) have run into issues where one mission didn't track as completed properly. If this happens to you and you're not sure which mission needs to be redone, check your progress on this site
, it actually shows you which missions you've done (that link shows you Heroic progress, but as Legendary completion gives you credit for Heroic completion as well, it should still work unless for some reason you've already run through the game on Heroic)
This achievement is more tedious than it is difficult because most missions can't be failed and you’re allowed to use Acrophobia/Bandanna. However, there are three
failable missions (and another mission with a major bug), so I'll share tips for those.Ep.5 Ch.1 ("Spartan Miller")
After fighting a few waves of Crawlers and Watchers at the start, you'll have to defend two research stations against waves of Prometheans. Losing both stations will fail the mission.
- TIP: this section is time-based, not wave/kill-based, and a new wave won't spawn if there are at least two enemies alive. All you have to do is leave two Crawlers alive and wait for about 5-6 minutes (you’ll know it’s progressing based on the dialogue). Even on Legendary, the Crawlers shouldn't be able to destroy both stations before the section ends
Ep.5 Ch.4 ("The Cauldron Base")
- There’s a separate achievement tied to this section called “No Easy Way Out” (10G). You have to keep at least one station alive, avoid pressing the third button after the defense section ends, and then clear out every wave of Prometheans that spawns. The 2-Crawler strategy can help you get that achievement too.
Similar to 5-1, this mission features a generator defense section near the start that results in mission failure if you lose both.
- TIP: There’s no way to cheese or skip this part, but it’s pretty easy because you’ll have a Mantis, automated turrets, and Marines. If you're having trouble here, try using a loadout with the Wheelman perk
Ep.7 Ch.1 ("Backup")
- You’ll get an extra achievement, “What Power Outage?”, if you manage to keep both generators alive (10G)
In the middle of the mission, you'll have to pilot a Mantis to defend Murphy's Phantom from an enemy Phantom. You fail if Murphy's ship drops to around 1% health.
- TIP: You don’t have to destroy the enemy Phantom to successfully protect Murphy, you can win by simply disabling all of its weapons. If you kill the Elites manning the plasma turrets on the sides and destroy the chin-mounted heavy turret attached to the bottom of the ship, it will immediately fly away and you’ll clear that part. A single well-placed Plasma Grenade is enough to do the trick
There's also one more mission that's supposed to be impossible to fail but has a game-breaking bug dating back to OG Halo 4 that can make it unwinnable:Ep.3 Ch.5 ("Hunting Trip")
Near the end of the mission, Covenant drop pods will start dropping onto the cliffs surrounding the map. It's possible for one of the drop pods to glitch out and spawn an Elite inside of a cliff
. This can make it impossible to win as the mission won't finish until every enemy is dead.
- TIP: I've seen people online say that it's possible to kill the glitched Elite with grenades or a perfectly angled Beam Rifle shot, but I didn't have any luck with that. If this happens, you will probably have to restart. I'm not sure what triggers this bug either.
- Warning: The first time the cliff bug happened to me, the Elite eventually fell out of the map which allowed the mission to end. However, the completion didn’t register in Xbox's tracker. MCC's menu claimed that I beat the level on Legendary, but the Smooth Operator progress tracker was 2% below what it should've been. My progress wasn't fixed until I beat this mission without the cliff bug happening. If this happens to you, make sure you check the achievement progress afterwards
On that topic—I strongly recommend using Xbox’s achievement tracker overlay (or whatever Steam’s equivalent is) to monitor your progress. Spartan Ops is notorious for sometimes failing to track completion properly. If you don’t check your progress after each level, you won’t know which mission to redo if one of them doesn’t track properly