You'll begin this chapter in an area with a gate on your left and a grapple point in front of you. The grapple point does nothing right now, so don't bother with it. There's a Sword on your right if you want to grab it. Go through the doorway on your left, down the stairs and pull the lever (Green Chest here), which will drop a ladder and close the gate you passed through before.
Climb the ladder and drop into the area from before, where you can now use the grapple point. As you're doing that, a Cyclops shows up, so get ready for battle. The Cyclops' attacks are powerful, but they're rather slow and can easily be avoided by dodging away from it. Keep attacking it in between its own attacks until the white bar appears over its head. Grapple it and Kratos will ride on its back, allowing you to attack the enemies that spawn with the Cyclops' club. Once you kill them all, Kratos will kill the Cyclops.
With the Cyclops out of the way, grapple the chain again and move backwards to open the gate. Go through it again and climb the ladder, then hold R1 and jump to the ledge on the other side. Open the Red Orb Chest, then use the next grapple spot combined with Square at the right time to break through the wall.
After a short scene, you'll be fighting the infected arm again. Avoid the sweeping attacks and keep hitting it until you get some green orbs. At this point, you can grapple the wall above it and bring it down on him. You need to repeat this again, only it will be on the other side of the arena, but note that it will summon some of the bugs to get in your way, so make sure you clear them as fast as possible.
Once you bring the second part of the wall onto it, the third stage will begin. It will summon bugs again, but its attacks are a lot easier to manage. Whenever you see one of the hands glowing, dodge to the complete opposite side of the arena. It will hit that hand on that side, as well as hit the middle with one of the claws. Keep attacking it when you get a chance until you can grapple it. This will start a QTE that will finally kill it.
You'll be "transported" to a new area, where you'll find two Red Orb Chests, as well as a spot where you'll learn that the prison you're in is a creature named Aegaeon. Pull the lever to open the gate, where you'll find three infected hounds. You can easily dispatch them by grappling them and throwing them at the grinder.
Before you grapple the grinder, go to the stairs on the left side and jump up to a ledge with a Red Orb Chest. Go back down and grapple the grinder to make the elevator start ascending. When it stops, you'll have to fight a bunch of the infected hounds, so do as you did before. Once they're dead, go to the clamps on both sides and lift them up to start the elevator again.
It will stop again and Megaera will toss something down to jam the grinder. You'll have to fight off a few more hounds, then you can grapple the wall and have to start climbing. You need to hurry, as the grinder will start moving as you're climbing. Use the wooden structures, jumping in between them until you reach the top. Grab the Green Chest if you need it, then climb the ladder to finish this chapter.
