You do have to wonder just how much Madam Jin is going "I really see what Ziyuan-jie meant about him, goodness" while WWX is battling with that spirit-resonant rage in front of her.

I now posit that WWX developed a light-touch-redirect of resentful spirits, one that uses his remaining qi (which is obviously not a small amount!), out of or on top of his initial approach, which was internal-channeling-redirect, which basically draws in resentful energy as if it /were/ his qi. The latter, I theorize, is the more brute force approach, but also the more certain. It clearly /is/ the one that does actual damage to him, though.