In his universe, he is the eldest of the three most powerful gods. Olimpos is a mixture of philosophy and myth. Hades represents death. As such, in his world, he is inescapable to all mortals. They cannot fight or wound him since he is an inevitability.

Now the story might be different where his fellow gods/immortals stand because he isn't a fixed concept for them. Which is why I say the Valar and Maiar could possibly wound him.