his/her weak point
It's his weak point.
Cercherò il suo punto debole mentre combatto!
I'll look for its weak point while I fight!
— Demon Slayer - Il treno Mugen (2020)
Se non le dispiace, potrebbe dirci il suo punto debole?
If you don't mind, could you tell us your weak point?
— No Other Choice - Non c'è altra scelta (2025)
That is his weak point.