Move along the path until you reach the Pits. Go down the rocky path on the left. Some undead will attack you, but they're nothing to be worried about.
There's a gap at the end of the road. Jump over it and approach Gaia's hand, lying in the distance. This way you will initiate a fight with Cronos.
At the beginning you will go through some QTEs. Once you find yourself between the Titan's fingers, quickly mash Circle to avoid getting crushed. Afterwards you will have a couple seconds to blind your enemy - equip the Head of Helios, charge it and release a beam of light.
You will be falling down. Press one of the basic buttons to land of the Titan's hand. Kill a couple undeads and prepare for some climbing.
Cronos will change position and you will have to climb up his arm. Jump over the gap and eventually you will reach a blemish - hit it a couple times to enrage your enemy.
He will set his hand horizontally again and you'll have to fight some more undeads. Additionally you will have to look out for the Titan's breath (it can take away quite a bit of health).
Fight the enemies until Cronos puts his hand by his arm, which will allow you to damage his fingernail (hit with the Cestus or use the Rage of Sparta). Once you deal enough damage (you might need to repeat the process several times and wait for him to close in his hand again), activate the QTE and rotate the left analog stick.
Cronos will begin to attack with his whole hand, trying to crush you. Try to position yourself between his fingers, which will allow you to climb up onto his hand. He will then move the hand to his head, which will allow you to blind him for the second time.
As a result you will be fired into the air. During the flight you will have to press one of the basic buttons to land on the Titan's torso and begin climbing up.
After a short climb you will have to jump and hit a grapple point.
If everything goes well, you will land on a ledge, where some undeads and two statues will attack. Killing them won't be too hard. Once they're history, go to the middle part of the part of the stone circle and open it by mashing L1 and R1. Afterwards begin attacking the tissue underneath.
After a couple hits Cronos will grab you with his hand. Climb it, kill some basic enemies and the Titan will try to crush you. Quickly tap Circle to free yourself.
This way you will end up hanging from the bottom part of his hand, above a pit. Go right and perform four jumps with grapples.
In result you will land on his arm. Begin running to the right (while avoiding enemies and jumping over stone barriers). Cronos' hand will be ramming everything in its way.
You will arrive at his shoulder, on a small arena. There's also a Cyclops. Hit him a couple times and take control over him. Once you do, begin attacking the spot from which the monster appeared (the cut arm). After a couple hits the Titan will try to knock you off. A QTE will start - press one basic button and quickly tap Circle.
As a result, you will find yourself inside the Titan. Begin running away from the deadly liquids. Move downwards quickly (by holding down R1) and avoid the obstacles by jumping sideways.
The camera will exit to the surface and so should you. During the QTE you will need to quickly press Circle, press both sticks at the same time and then tap Circle a bit more.
This way you will get out. Walk along the fingers and glide towards the stone ledge on which you were fighting a moment ago. Once you get there, destroy the crystal chain.
Once it breaks, another QTE will begin. Press two basic buttons to finish it successfully.
This way you will arrive at the head. Destroy the spike fragment and move to the forehead. A QTE symbol will appear - press L2 and R2 at the same time and watch the Titan die.
After you land, destroy the crystals in front of you and jump into the hole behind them. This way you will arrive at the Forge for the last time.