For being the drunken lout of Cel's old gang, he may be the toughest warchief fight. You can get his attention by destroying three grog stacks. The best way to get the optional objective of hitting 10 orcs with a single blast is to jog through the fortress, attract a train of followers, and then shoot the stack.
Koth, when he appears, is arrow proof and fire proof, and like any high-level warchief he'll eventually adapt to stuns and vaults. He is vulnerable to grabs and throws, but you can't throw him off buildings. You'll just have to wear him down with basic swordwork. When he breaks, grab him and Dominate him for the second bonus objective.
This is easier said than done- you have to make sure no one stabs or spears you while you work. Best bet is to stage this tete-a-tete on a rooftop. When the domination is complete, the mission ends.