I mean, he is immortal. He experienced pain but his cells just went back into the planet and came back in Advent Children, where he "evolved" once more with the wing and, as per the novellas, now knows how to travel along the lifestream through time when before he only used it to travel across space. That timeline the OG saved is obsolete now as he resets to a point in the past, equipped with information from the future. How he attempts things this time remains to be seen, but it would appear the heroes are doomed to face him forever unless they can find a way to stop him at the source (his cells traveling along the lifestream).