If it was that easy to defeat King why didn't Zoro's ultra tiger hunt attack hurt King?
Why did Zoro continue to use ACoC against King after learning about how his body works?
King's defense doesn't disappear just because he's not using his flames. It just decreases.
King was Zoro's Katakuri but much stronger and harder to deal with. Zoro had to obtain a new power to beat him.