I performed 700+ assassination animations against the elite at the beginning of the mission Pillar of Autumn, trying different positions of the elite, different in-air sprint timings, time waited after the checkpoint reloads, different elite classes, etc. These attempts were more or less equally done on my PC and Xbox One X.
I then made two accounts, “Please 3436500” and “Please 343 2702” and successfully completed the challenge on the second try with the former, and the first try with the latter. The accounts have 8 and 2 minutes of playtime on Halo MCC and had no issues earning the achievement. Combine this with the fact that I have tried 700+ times on my account “Tyrael” goes to show that this achievement is legitimately bugged and not “finicky”.
I then had three friends who got the challenge very quickly on their accounts (within 5 minutes) download my account onto their own xbox and attempt completing the challenge on my account. After both tried it for an hour each with many successful attempts, doing the same methods in which they completed the challenge (on Pillar of Autumn) none of them were able to get the challenge to unlock.
Because of all of this I think that there could be something associated with a player’s account that actually causes the challenge to not register, however I have no clue what aspect that could be. I have tried using different controller layouts, different armor configurations, all in an attempt to find the issue, but I feel it is more deeply rooted than that. Here’s to hoping 343 actually looks deeper into this issue…