When you reach the Challenge of Hades, it can be a bit tricky figuring out what you need to do. A glowing ring with four empty circles will appear on the floor to the left of the huge statue in the center of the room, and a bunch of centaurs will charge in to attack you.
What you need to do is kill the centaurs while they are inside the ring. There are a couple of ways of accomplishing this. One way is to use power attacks to stun them while they are inside the ring so they get the "circle" prompt over their head, and then kill them with the QTE. Another way is to weaken them with physical attacks, and then use Poseidon's Rage on them to kill them inside the ring.
Once you kill a centaur inside the ring, one of the empty circles on the ring will light up. Continue to kill centaurs in this manner until all four circles are lit. At this point, another ring with another four empty circles will appear on the other side of the statue, so run over there and repeat the whole procedure again.
When both sacrifice rings are completed, the way will open for you to complete the puzzle.