I'm actually okay with it in this instance, because it's a technique they developed as a specific response to that specific attack that they'd seen before.
They showed us the attack in the beginning of the arc, showed us how effective it was, then gave the heroes a little off-screen down time. It makes perfect sense to be that they would develop a counter for it.
It's less acceptable to me when the hero just happens to have an ass-pulled new ability that's the perfect counter for an attack they're just now encountering for the first time.
I guess kill him instantly with a single instant death attack? Which is a lot harder to do now that there's two of them.