They really need to know about his ability though and a bunch of dedicated assassination. For example, since people NORMALLY died after getting shot through their heart or loss of blood, people would definitely assume that Tatsuya is already dead with that. Of course we know that he's going to survive that, and by the time those bunch of morons that tried to kill him in conventional ways realized that... well, they're already dead
.
It's a good thing that his restoration ability is a secret
People that'll try to kill him wouldn't plan a specific way to destroy him since they thought he's just a normal human.