couldn't kill (someone) if you tried; tough as nails
A hyperbolic set phrase meaning someone is so tough or resilient that even an attempt to kill them would fail. Often used figuratively about a person's tenacity or indestructible nature.
That person is the type you couldn't kill if you tried.
He has a toughness as if you couldn't kill him if you tried.
A set phrase composed of 殺しても (even if you kill) + 死なない (does not die). The exact origin is uncertain, but it is a common hyperbolic expression in modern Japanese.