How to get the black bar iron quickly in a duel between Douro Mainland Soul Masters?

Douluo Biography is an original plot level based on the original work and the past events of important people in Douluo story as the background. The biography of Dou Luo is divided into several chapters. With the passage of opening time, each chapter is an independent story, and each chapter contains several sections, which are gradually opened with the passage of opening time. Each lesson has three difficulties: ordinary, difficult and hell. You can get a reward for the first customs clearance. When each chapter is first opened, the corresponding challenge activities will be opened. During the opening period of the challenge activity, you can get additional activity rewards through customs clearance. After the challenge activity is closed, no additional activity rewards will be obtained.