This one uses multiple skills to achieve full success. Your primary goal is to make fifteen orcs kill fifteen other orcs- easy enough with the Dominate power.
Your first optional objective is to have three orcs hit while pinned. This is straightforward as well- just Dominate one orc in a pair and Pin the other. Rinse et repeat.
The last objective requires slightly more finesse, and a thorough reading of the guide. What the objective means by 'combat domination' is using the Consume power. Simply dominating while in combat, or dominating hostiles knocked to the ground, or dominated hostiles while you're grappling them do *not* count toward this objective. Instead, build your Might with some melee fighting, then Consume an opponent.
Because the melee build-up sometimes takes longer than you expect, it may be wise to kill your other Dominated servitors first, so they don't accidentally complete the mission while you're still working on your third consume (note also that there are an infinite number of orcs available here, so don't worry about using them all up in your quest for Might). Once you've got it, and your pin hits, dominate about half of whoever's left, and let them take care of the rest for you.