The fight against Miejue. ZWJ didn't know about his internal, so theres nothing to let down on. He didn't use his internal to block her palms, which would've dealt the damage with or without the "tricky" bit.
Bit remember who ZZR is. She was engaged with ZWJ and ZWJ "humiliated" her in front of the whole of Wulin by deserting her for Zhao Min. ZWJ feels too much in debt to ZZR and its natural not wanting to strike her down.