On this page of our guide to Psychonauts 2, you will find a walkthrough for the last boss.
The last stage of the game begins in a scary forest. It will lead you to the ultimate boss. You have to watch out for the sharp bushes and the whirlwinds.
If you haven't bought upgrades or healing items, this is your last chance to stock up.
When you get to the monument, you have to interact with it. That way, the thoughts will appear. You have to jump on them and get to the surrounding platforms.
Your task is to reach the grandparents' bedroom. There, you will learn the truth that will allow you to go forward.
To get back to the arena with the statue, you have to burn a painting with Maligula. That way, you'll get you to the thoughts that will transport you to the arena with the last boss.
A description of the boss-fight can be found on a separate page of the guide. This stage ends the game.