From what I remember SE stated before launch that Odin was meant to have a special mechanic where each time he's defeated, part of his memory returns to him which actually increases his difficulty in the next encounter, meaning potentially he'll reach a point where he'll be so strong as to be virtually unkillable (although as I understand it Yoshi mentioned that he would actually reach a 'cap' in difficulty so as to possibly avoid such a situation).

I can only guess that it's possible that he's reached this point sadly, assuming SE did in fact use this system - I have no idea if it's true or not though.