Well, it is anime. unexplained increased durability just seems to be the norm for plot reasons, a lot of the time because you both need to show this attack is cool and effective, but this guy needs to survive, but you don't have any better explanation for why other than "look they are really determined and have a strong will to survive okay?"

I'm not saying we can't include their existence. I'm just saying that I don't see the existence of what is basically DBZ ki attacks as necessary or desirable even with what your talking about. there are more ways to kill people than directly shoving your shiny attack into their face, thats kind of the entire point of being an assassin in the first place. that and the highest rank you can get is S-Rank so if there is any S-Rank durability stuff, it can't no sell any offense that is also A/S-Rank, and those S-rank offenses don't HAVE to be some blasty chakra thing. there can be S-rank poisons, S-rank taijutsu or weaponry, or S-Rank traps, things like that. I'll think as far outside the box as I need to, and come with as many counters and such as I can, to make sure some stupid latent super-durability doesn't screw this up into a DBZ-like direction.

as for Papersengan, thats a better idea than what I had for it, honestly.