I feel like the goddess is underestimating him. If she knows he came back in time then she should be VERY wary of him. Because whatever caused humanities extinction, was beaten by him. Not her, not her followers, not any other gods, but him. And honestly, he shouldn't need any help. As far as we know he had no help doing it all the first time, aside from his companions and their fighting power, so it stands to reason that if he simply expanded whatever training and effort he did the first time around then everything should work out pretty well. This is kind of the fear i had with this story, we know how it ends. We know he is the strongest, since he ends up beating the bad guy in the past. It's just he went back in time to do it better and not lose so many people. So realistically, as long as he does whatever he can to get back to his former power, none of the enemies should pose much of a threat, especially since he knows how all of them fight now, having done it once before.