First, there must be some kind of power that was strong enough to slant the mountains. That was easy. Xuan Bing herself could do it. It wasn't actually a difficult thing for her. In fact, even Han Bingxue could do it if he tried hard enough.
However, that was the first step!
The next step was the real challenge. The two mountains had to be slanted at the same time. It wasn't difficult for Xuan Bing either. She just needed to work a bit harder. However, the two mountains had to lean on each other perfectly.
That was the most difficult one, but Xuan Bing still could do it.
The real problem was that the two mountains had to prevent collapse. Neither of the mountains could collapse during the process and the countless days after it was finished!
That was the real problem!