So, I have been working on all my Halo CE/A Campaign achievements and this is the ONLY one that wont pop for me. I have tried it multiple times on every difficulty, with and without bandana on. I even did it on Co-Op with my wife signed in and the achievement popped for her instantly while mine did not. I ran through again with myself as the primary profile and hers on secondary and still, no achievement for me.
I even make sure to hit all the checkpoints on my way up to Keys’ room just in case and I simply cannot get this achievement. It’s the very last one I need to complete all the CE campaign.