Two things I found on TV Tropes about the Elden Beast.
"Where does it sit in regards to the Outer Gods? Is may simply be the equivalent of one of their chosen servants, like the Lord of Frenzied Flame or Malenia, especially as it's described as a vassal for the Greater Will. However, there are arguments to be made that the Greater Will is
an entity superior to the Outer Gods or, alternatively,
a mostly passive, if intelligent in some way, force of creation that existence originated from, so commanding a being equal to that cosmic group wouldn't be unexpected. The fact that at least two powers gifted by Outer Gods are needed to burn down the Erdtree, which it's connected to, further implies the Beast compares to them in terms of power itself, raising the possibility that it may actually be of the same nature as them - as a matter of fact, being the embodiment of the Elden Ring is similar to the Outer Gods being the true natures of things like the Scarlet Rot, such as the reports of a god buried beneath the Lake of Rot.
"
"
Brought Down to Badass: Twofold, or even
threefold going by separate means that weakened it. By the time the Tarnished can enter the Elden Throne, the Erdtree is already set ablaze using the Flame in the Forge of the Giants, and ravaged with the Rune of Death. As the Beast is connected to the Erdtree, its power and influence are almost certainly affected. And because it is revealed to
be the Elden Ring itself, Marika shattering it and her children claiming its shards long ago logically must have weakened the Beast to a certain degree, and it has a noticeable wound on its chest as a visual indication of this. The fact it's
still powerful enough to serve as the final obstacle to the Tarnished at the height of their own strength despite these factors speak more than enough about its power as a god."
I kind of remember we already discussed this some time ago but yeah, the general implication you can piece together is, by the time the Tarnished fights it, the Beast has been weakened and is not at its prime.