The Xuanyuan Dharma King's four major disciples each had the strength of a Sect Hierarch, and among them, the Five Lascivious Venerable was the strongest person Shi Feiyang had encountered after Elder Kuzhu and One Light Master.
Back when he fought Elder Kuzhu, he was only facing his reincarnated body, whose Daoist power was far from comparable to that of his original self. Moreover, Shi Feiyang had preemptively calculated and exploited the flaws in his cultivation, causing his original self to become fossilized in meditation. Only by inducing him to fall into Demonized Cultivation did Shi Feiyang manage to secure victory.
Later, when he fought One Light Master, it was also because the latter was nearing his Heavenly Punishment, his mind in chaos. He had first caused the myriad demons employed in the Master's formation to turn and devour their owner, also driving him into Demonized Cultivation and partial paralysis before striking.