How do I get the relic in Thors eye at Midgard Serpent?

I had this problem and the only solution I found was to try falling (or jumping) down onto the top of his shield. You'll miss/die a good few times, but find the right spot and it *is* possible. From the shield you can walk onto his hand and then jump to one of the pillars in the centre.
As an aside, I gather the "correct" method to get onto his hammer is to jump across from the other two pillars, but what's easier if you've landed back where I suggested is to jump to his chest (where the belt was), and then scrabble up onto his hammer-arm from there ("inside" his arm, rather than "outside" where the pillars are). The camera goes a bit crazy and it doesn't look like you're supposed to do it, but it does work. Good luck!