I’ve played with over ten different players in co-op and the progress for this achievement is 100%, but it refuses to unlock.
Little help?
Playing on the Xbox One version 
I’ve already tried power cycling, played over five games with different players since hitting 100% progress.
This is a common Xbox One issue that can happen with any game. See the first question on this official page. Up until the past few weeks, the achievement would unlock on its own within a day or so. But lately I’ve seen reports of several days passing and an achievement still doesn’t unlock.
BTW, today I learned a trick for checking all-or-nothing achievements that don’t show 0% or 100%. If you Compare Games in the Friends app, you should see “Locked (100%)” for any achievement that should have unlocked but didn’t immediately do so.
Currently, I’ve got 4 Guacamelee achievements in this state from last night and today. The problem seems more common than usual today. Perhaps it has something to do with yesterday’s service outage that prevented any Xbox One achievement from unlocking. Perhaps the system is still catching up. Hopefully we’ll all get our achievements soon.
Have you tried clearing your cache?
Hello The Little Moa.
This appears to be a glitch with the Xbox One Achievements App, as MathGuy42 explains. I have seen these glitched achievements take anywhere from a day to 6 months to unlock, and about the only way to get them to unlock is to periodically check the Achievements App and be patient. Sorry we can’t help you further.
> This appears to be a glitch with the Xbox One Achievements App, as MathGuy42 explains. I have seen these glitched achievements take anywhere from a day to 6 months to unlock, and about the only way to get them to unlock is to periodically check the Achievements App and be patient. Sorry we can’t help you further.
This issue is ONLY happening with Halo: Spartan Assault though. I’d understand if it was happening for other titles, but since this achievement has failed to unlock I’ve managed to unlock multiple on Titanfall and Watch Dogs.
Maybe you personally can’t help me out, but I’d appreciate a response from the Forum Team at least.
> > This appears to be a glitch with the Xbox One Achievements App, as MathGuy42 explains. I have seen these glitched achievements take anywhere from a day to 6 months to unlock, and about the only way to get them to unlock is to periodically check the Achievements App and be patient. Sorry we can’t help you further.
>
> This issue is ONLY happening with Halo: Spartan Assault though. I’d understand if it was happening for other titles, but since this achievement has failed to unlock I’ve managed to unlock multiple on Titanfall and Watch Dogs.
>
> Maybe you personally can’t help me out, but I’d appreciate a response from the Forum Team at least.
Yes, it is with Halo: Spartan Assault in this case, but I’ve had friends who went through Spartan Assault on the XB1 flawlessly, but had the same glitch you are referring to on other Xbox One titles. I can let 343 know, but I have a feeling they’ll tell you the same thing.
One thing I can suggest you try is unlocking a separate achievement in Spartan Assault and checking to see if this one unlocks in the next few days.
Moa, did you try clearing the cache? I know @XboxSupport has suggested that before.
Lycan, I’m curious what procedure Xbox Support is giving for clearing Xbox One cache. There isn’t a menu option for this like on Xbox 360. These instructions claim to clear the cache, but they aren’t from Microsoft.
I had 4 achievements in Guacamelee reach 100% on Thurs and Fri with no unlock. I redid the requirements yesterday and they unlocked. Someone else had mixed results, with some previously glitched achievements unlocking and some not. Regardless, after Thursday’s Xbox One achievement outage, perhaps some improvements are being done on the backend. I’d try playing with another stranger and hope for the best.
> Lycan, I’m curious what procedure Xbox Support is giving for clearing Xbox One cache. There isn’t a menu option for this like on Xbox 360. These instructions claim to clear the cache, but they aren’t from Microsoft.
I don’t know. It was on Twitter. I just saw that they suggested that someone clear the cache.