@Strax_Lycaon so thing about the heavenly flame, is with the flame mantra, his dou qi method and rank rose up, so his level is higher than it was before swallowing, which they kinda gloss over... heavenly flames are an entirely different class from beast flames as beast flames are to a dou qi flame. So if a beast flame lets you fight upper levels of your own class, heavenly flame fights across classes. it's explained better in the beginning where it covers the dou qi method, dou technique, and level of practitioner. Of equal level, technique decides it, of equal tecnique and level, qi method decides it. The flame mantra evolves with every flame it consumes, so it evolved from huang (yellow) low to xuan (black), not remembering specific level, when it had the flame consumed. The point is he's got "di" class techniques with a heavenly flame, and most everything below the dou wang class easily. the space between dou shi to dou wang is very different from dou wang to dou huang, so as the class increases the gap widens massively.
TL;DR -> Heavenly flames are something even a dou huang would have trouble resisting, anything below that is firewood basically.