It was already hard enough to deal with a single Qingtian Emperor, and dealing with two of them simultaneously was nearly impossible. As such, Zhang Xuan had slowly made the necessary preparations. First, he had his clone conceal himself in the hall before retreating again and again so as to make the Qingtian Emperors lower their guard, as well as to lure them where he wanted them.
The final explosion served as a distraction for the duo, buying sufficient time for his clone to launch an assault.
It sounded simple on the surface, but it was actually much more complex than it sounded. In order to put up a convincing act to fool the Qingtian Emperors, he had needed to carefully decide which techniques he should use, how far he should escape, where he should stop, and from which angle his clone should launch his assault to not be noticed.
It was only due to such careful calculation that the plan worked out in the end.