Originally Posted by
Balipu
"Ravana also asked for absolute invulnerability from and supremacy over gods, heavenly spirits, other rakshas, serpents, and wild beasts. Contemptuous of mortal men, he did not ask for protection from these. Brahma granted him these boons in addition to his 10 severed heads and great strength by way of knowledge of divine weapons and magic."
In game this was handwaved with Shiva not having enough aether when fighitng Ravana, plus his inexperience with fighitng other primals, but there was no way anyone but a mortal could have defeated Ravana.