Here is just something thats mostly common sense but I figured I'd post anyway.
Generally groups check if you have the daily done, usually through checking the quest log. So if you want to repeatedly farm the boss, what you need to do is simple.
Get two or three friends, one preferably being a warlock. Make sure that neither of you have completed the quest that day so your victims dont suspect anything.
Ask random people In LFG, summon them, get them to summon the boss, and kick them when they're done. Rinse and repeat till you get your desired mount
Enjoy
EDIT: kick them right when they start it and just tank it for a minute so they port out and dont get to roll on the loot.