The humans would win. Think about it. What resource the the humans use that none of the other races use... Horses. One human would be riding along on his horse, and a Nephil would shoot a bow at him, a slith would try to spear him, etc, etc, an they would miss and hit the horse. The horse, being indestructible, would not be injured, and the human would escape. He would then tell all the armorers to start making horse-hide armor and equip all the soldiers with it, and voila! Indestructible human soldiers.
They could even make horse-hide robes for the mages!