It is some how possible to get all of the mounts for 10 an 25 man garrosh kills achievement while not having a garrosh kill. Proof is you can simply go look at blood legion armory which is up to date since they have all logged out since raiding or you can look at a couple other guilds that have 13/14 downed but have mounts for 14/14 10 and 25.
I know it exists but I'm not sure how to do it myself so if you can figure it out before it's hot fixed, cheers on your free mount.
Sorry this isnt a how to guide, but i did state that in the in the title.