A Warchief must be drawn out before he can be dominated or killed. Each Warchief requires a different tactic, which can be learned by taking on his Captains. If you kill a Warchief, you will gain 70 power points.

Warchief	How to unlock
Cry Havoc	Fighting and killing waves of enemies will force a Warchief to come out of hiding and fight your.
Dishonored	Humiliate a Warchief by forcing his minions to abandon him. Wraith powers can be used to scare away the Uruks from their strongholds and abandon their leader.
Killing Spree	Rapidly slaying a large number of Uruks will draw out their Warchief.
Riot	Use the Dominate ability to force Warchiefs to start a riot against a hostile Captain or Warchief. You can use the chaos this causes to kill the target easier.
Strike From Above	Kill yellow marked targets by surprising them from a high vantage point. This will force the Warchief to face you.
Taunt and Kill Supporters	A Warchief can be forced to fight you by taunting him first, then killing a number of his supporters without taking any damage.
Uncaged	A Warchief can be forced to fight you if the caged Caragors in his stronghold are freed.
Vantage Point	A Warchief can be ambushed by stealthily infiltrating his stronghold then surprising him by attacking from a high vantage point.
Warchief Supporters	Execute the yellow marked supporters of a Warchief in open combat to draw out him out. Ground or Combat executions are the best way to get the Warchief's attention.