Yes... Xiao Feng is certainly a better fighter than Zhang Wuji, yet Xu Zhu is just ridiculous. Not only does he have enough inner power to glow in the dark, he also has all manner of mysterious Xiao Yao Sect arts which even Murong Fu can't learn and which are definitely not inferior to any skills that Xiao Feng has. Sure, XF may be more skillful in general fighting ability, but that doesn't compensate for a 300% difference in inner power?!
JMZ was aware of how much inner power Xu Zhu had after he used Xiao WuXiang Gong to hit that bell, he had no confidence of being able to defeat him, yet when he punched Xu Zhu on the shoulder with Da Jing Gang Fist, he still expected Xu Zhu's shoulder to be crushed to powder, in fact Xu Zhu feels some pain but is undamaged. Then he kicked Xu Zhu 5+ times on the heart, the book doesn't even mention Xu Zhu feeling it. Then he hits Xu Zhu's arm with 2 chops which are enough to cut off someone's limbs, again the book only mentions JMZ's palms aching with deep pain and nothing about XZ feeling anything. And XZ, from a relative point, totally sucked at this time...