He is never attacked in canon. I think him being vulnerable to the Valar/Maiar would make sense though. Equal deities in their respective universes and all. Hades isn't a fighter as it stands. He's definitely more of a spectator.

The largest he ever appeared in the manga was like a big cow/bull. I would keep with that for his limitations.

Yes. Death is very different to everyone and that is what he represents.