Why would he wait for his body to be broken to show his trump card?
Really mind boggling why so many wuxia authors like this cliché stupid strategy of holding back trump cards until on the brink of death. The mc is going to use the trump card anyways... and sure one could argue that the timing is better, but that fails as well. A person becomes weaker and more limited the more injured they are.
In this case it's somewhat more ok as tang san's weakness is mostly eliminated because of his grass. But in general this cliché bs makes me wonder what's going on in the author's head by making their MC look like a dumbass. And even worse is how the author tries to build an image of how their MC is a genius with an intellect unparalleled under the heavens in most wuxia stories.