You face Griever at the end of “The Hollows”. There will be lots of those blue crystals that you will need to smash. If they get hit with his laser, they will turn red and you will have to wait till they cool down and turn blue again. Once they are all destroyed, take that train car push it up and then charge your Tremor Gauntlet and smash it into his knee. He will fall down and you can rush in and smash his blue crystal. When he pushes you back, he will smash the wall to make rocks fall, so just run around and dodge. If you break off a little early, you can run down the opposite track as the car. If you run just in the tunnel, you can push it back before he starts to drop rocks, stunning him again.