>>96560196 (OP)
Do you want a weakness like
>A trick that can be used to defeat it without fighting, bypassing it as a foe or threat entirely
>A weakness that is the only reasonable way to injure it, making fighting it without the weakness suicide
>A weakness that is a way to hurt it badly (but it's still vulnerable to regular extreme violence)?
For the first, the "make it drink wine" thing someone suggested ITT is neat. It's a venomous creature with a human face, so it can succumb to human passions and its own bestial venom. Being overcome by its own opposed natures is poetic.
For the second, the only place it can be cut is on the belly, and the only things that can cut it are the talons or horns of [insert rare beast with large talons or horns here]. Something like a wyvern's talons. Manticores live far from wyverns because they are natural enemies and fight to the death if they meet. Landing the blow will be difficult, and many find that they are stung fatally in the process of cutting it, joining the beast in death.
For the third, as a creature of spite and poison and thorns, it hates the true comfort of faith. Any religious litany chanted in its presence will make all weapons one degree stronger against it (increase die size or inflict an extra wound or whatever), but it will either shrink away and cower from this effect or attempt to savagely maul the praying person(s), so taking advantage of this is difficult.
