Complex sentence: When fighting gods Atum quickly reverts to God Eater form, and while possessing incredible strength, stamina and flight is not totally invulnerable as it has been hurt by divine weapons of exceptional quality.
Split into simpler sentences: When fighting gods, Atum quickly reverts to God Eater form. While possessing incredible strength, stamina, and flight in this form, it is not totally invulnerable as it has been hurt by divine weapons of exceptional quality.