In the past few days of combat, the actual motive was to prevent him from breaking away from the battle, and then to assassinate officials of the Sealed Emperor's Palace. With each one killed, the Sealed Emperor Great World weakened by a measure.
To kill Emperor Luan... one had to first sever the Sealed Emperor's Kingdom's fate, which was a thriving National Fortune Divine God Dragon. Cutting down the nation's fate was akin to slaying the entire Sealed Emperor Great World.
An almost impossible mission.
To kill Emperor Luan, one must either destroy the Sealed Emperor Great World or slay him outside of the Sealed Emperor Great World, where the National Fortune Divine God Dragon could not manifest.
In fact, the best approach would still be for the Sealed Emperor Great World to descend into chaos on its own.
Regrettably, a hundred and ten thousand years ago, the Mystery Huang World failed to seize the opportunity.