The Lord of the Universe was exceedingly clever. The moment he saw Chi Xue and the others setting up a formation beside the one already taking shape, he understood what they were planning. Naturally, he wouldn't simply stand by and watch them set up a large number of formations, so he controlled the Nine Nether Nether Soldiers and launched an attack to stop them.
However, many of the Nine Nether Nether Soldiers capable of arranging Buddha Kingdom Worlds or skilled in firing multiple Impact Arrows had been killed earlier. They couldn't be recreated and sent back to the battlefield so quickly, so the Nine Nether Nether Soldiers could now arrange far fewer Buddha Kingdom Worlds. The same went for the soldiers skilled in firing multiple Impact Arrows. Chi Xue and the others could likely intercept all their attacks with help from Little White and the other Bone Soul Extremes, giving Ling Tian's Clone and the other cultivators time to set up a formation—or at the very least, its initial framework.
As mentioned before, even a formation that was only a framework still had formidable defensive and offensive power. So even if a large number of Nine Nether Nether Soldiers skilled in firing multiple Impact Arrows or arranging Buddha Kingdom Worlds arrived later, they could still use the framework to hold them off and continue strengthening the formation as they fought.
Once this formation was in place, Chi Xue and the others could keep setting up more formations nearby. Their numbers would continue to grow. Then, even if more and more Nine Nether Nether Soldiers mastered multiple Impact Arrows, they wouldn't pose enough of a threat to force the group to abandon every formation. After all, everyone knew they could set up formations much faster than the Nine Nether Nether Soldiers