Lets do a bit of facts here, sense you all are basically not realizing a few things. Sephiroth from AC is Jenova. Jenova has survived a 30,000 megaton equivalant force opon crashing with the planet. She regenerated instantly. Kratos didn't survive being stabbed once.
As for the "god" thing. Gods are portrayed much differently in both games, in GoW they are all really just titans, they can ALL die. Sephiroth was going to become a completely different being all togather.