I have used a Mangudai to destroy a Siege Onager several times, in several games, both single player and multiplayer, but I did not get the checkmark indicating completion. The Mangudai was the only entity that was battling the Siege Onager, so it landed the final blow.
I posted in discord as instrcuted, but received no response. Posting here, though now, the challenge is over, but none the less, I don’t want to have future issues. The single player games did not auto record the game that I completed the challenge. One of the multi-player games did record, but the one that did, I was a Elite Mangudai when I destroyed the Siege Onager (not sure if that matters). On the others, I was upgraded and destroyed the Siege Onager as a Mangudai.
I did try to upload the recording, but the site gives an error that the file type is not alowed.