Nanatsu said: Iku: "Hello Tenshi" Tenshi: "Hello Iku." Iku: "How's the hand?" Tenshi: "Still a bit stiff... Can you do it again?"
Iku: What was it like being stabbed? Tenshi: It was the single most pleasurable experience in my life... Iku: What was the second most pleasurable? Tenshi: *Looks at front* *Grins*
According to SWR, Tenshi's body is so tough that Sakuya's knives couldn't break skin. Iku here just up and drove a knife straight through her hand.
To be fair, Sakuya defeated Tenshi before Tenshi's storyline, and it couldn't have happened with Sakuya's knives being always useless. It might mean that her body hardening is an activated technique like Byakuren's buffings, rather than a continuous thing.