Aku can be hurt by any attack that is Divine or mystical in nature. That is why jack's Sword is able to harm Aku. Odin, seth and a third god (whose name i forgot) Drove Aku to earth and defeated *him*. The Elementals from Mafia jack also defeated Aku.
I think that was Shiva and actually they thought they had destroyed Aku entirely. The only thing that made it to Earth was a mere fraction of Aku's total form and it took him millions of years to regain his strength after that. Aku was like a cosmic darkness.