It was mentioned two times (when the Titan Sea Class was introduced and when Ferocious Beasts were first introduced) that Ferocious beasts have a life rating even higher than dragons. Ancient Fiends on the other hand are "no match for dragons in terms of life rating but fully capable of rivaling Ferocious beasts(which are supposed to have an higher life rating than Dragons)" [Chapter 1849]. Is was also mentioned that the Ancient Earth Serpent (an apex existence even among Ferocious Beasts) does have a life rating "now weaker than a Dragon's". This is probably a typo since life ratings shouldn't decay with time, and it would make a lot more sense for it to be "it's life rating was not weaker than a dragon", considering than even average Ferocious Beasts are supposed to have an higher life rating than Dragons, let alone an apex one.
So, probably there is a mistake about Ancient Fiends and Ferocious Beasts, because it appears that Ferocious Beasts are actually stronger than dragons.
This would also make a lot more sense because ordinary Dragons shouldn't be capable of overcoming Tiers and Fighting against Gods, only divine and maybe golden Dragons should. (It was mentioned while speaking about divine Dragons that while ordinary Dragons were invincible amongst opponents of the same tier, Divine Dragons had the power to transcend tiers).