Essentially you simply rush Kushluk with your initial forces, but send one of your troops to attack the tower on the way to him to lure all of the red army (Kara-Khitai) to respond to that, giving you the time you need to break down the wall separating you from him. He will run away, so you will need to track him down. He should be near his Town Center in the south-west of the map, but be careful to avoid patrolling units as it is imperative that you take no / minimal losses in order to have the strength to kill him.