Instructions
Use the Wall Run ability to enter the second portal, which leads to the Living Forest. Use your Fist of Ruin to smash statues that appear as obstacles. At certain points you will receive messages to feed the trees' hunger. Throw Putrid Corpses into the trees' mouths in order to pass each area, until you reach the final area. Jump into the snake's mouth from the ledge to enter Reptile's Lair.
Smash the clay statues in Reptile's Lair to make your way to the arena, where you will encounter a sub-boss, the Giant Snake. Attack the pillar that the snake is coiled around with Fist of Ruin to smash the pillar and cause the snake to go to the next area. Climb on the rubble, then smash the second and third pillars. To reach the final pillar, long jump over to the platform it's on, which weakens the platform you're standing on, collapsing the tunnel on top of the snake. A short cut scene plays and you encounter Reptile.
Watch for Reptile's eyes when he's invisible and running around on the arena floor, so you know how to attack him. When he's damaged enough, he'll jump up to an inaccessible area and shoot fireballs at you. Dodge these fireballs, then attack him when he returns to the arena floor. When the Fatality prompt appears, do the Fatality Quick-Time Event to kill Reptile. Collect the Dragon Koin to get the Wall Climb ability.