Since that is the case, let's just skip straight to the very end. Merlin's smile grew more joyous.
He was actually eager to see where An Su's limits were, to find out if he truly had the qualifications to bear "Heaven's Gate"!
The Magic God Merlin gently raised his hand, the rotational authority of Heaven in effect within the man-made Nether, vast magic power surged wildly, the world immediately turned upside down as circuits went haywire. He slightly interfered with the historical process, bringing forward the surprise prepared for An Su in the sixth stage of the attack to just a little bit earlier.
It was all because of Cersei's scheming,
which directly reduced the six stages to three.
But fortunately, Merlin was a privilege hound.