he can be killed, but literally right after he dies from anything he regenerates and cant be killed by it again. he has already been killed by brute force, superhuman brute force, numerous different powers, natural things (starvation, dehydration, etc), the elements, bullets, stabs, being strangled, poisoned, the vaccum of space, the sun, and many many more things. which means saitamas super human 1 punch k.o. already doesnt work on him because he has already died from something like that. when you say "punch him into the sun" that wont work because doomsday can literally take a punch thatd send someone flying like that and still be standing in the same spot as he was before he was punched.