You'll summon this ancient warchief with three combat executions (using the Execution ability- regular ground executions and beast executions don't seem to count).
It's hard to properly Execute somebody by accident, which is good, because your secondary object here to to rack up a hitstreak of 25. This isn't too hard, but try to steer clear of ologs, and use arrows to take out spearthrowers and archers.
With that kind of might-building flow, you should have no trouble charging up for three executions, and drawing out the Champ himself. Unfortunately, he has that most despicable of traits:
The key to this victory is going to be on the rooftops, but we'll come back to that. First you'll want to top off your Might again by battling grunts, then sprint back and Consume the champion (as your second objective asks). Then you take to the roofs. The hoi polloi won't follow you here, and it leaves the Champ vulnerable to stealth attacks.
Stealth attacks daze him, and a dazed warchief is a soon-to-be-yours warchief. Cut him up, repeating the stealth attack a few times if needed, then Dominate him to complete the mission.