The abominable amalgamation was regenerating.
Extremely quickly, too, drawing up the lake's waters and recreating the hand it had lost within mere seconds. An impressive feat considering the hand alone was the size of a large building.
So, then, Li's theory was correct. The lake itself was a spawning pit, a mass of organic material with which Dark beasts could be easily created, or, in this case, used to regenerate this boss level creature.
The entity spurring this regeneration was basically part of the lake itself, so there also was no "core" to target as there traditionally would be in Elden World bosses that had annoying abilities like high speed regeneration that made them far harder to kill than normal.
And even if there was a single target he could hit, it was fused deep within the heart of the forest, and he could not damage the heart anymore than it had already been.