Your first objective in taking out this orc-of-legend is to free five caragors from their cages. Easy enough- there are certainly more than five cages to choose from.
You might as well fulfill the goal of three mounted executions right away though, before freeing all five. There's no time limit, and it's slightly easier to do this with no legendary captains breathing down your neck.
With that box ticked, free the other caragors to summon your foe (and his entourage).
Your second special objective is to ground dominate this guy, so keep that in mind for the endgame. Luckily for you, the endgame isn't very far away. Ol' Iron Mount here has the Soft-Headed weakness, so you can both neutralize him and deal massive damage with charged headshots. Find a high vantage and start sniping.
Use Quiver of Souls to keep your arrows topped up (works great with Shadow Strike Pull- you don't even need to leave your tower top to snag refills). When the Mount breaks, leap down, hit him 'til he falls, then use Ground Dominate to seal the deal.
Sealing the deal completes the mission.