Here is the full list of all 14 The Treehouse Man achievements.
Defeat the arachnid manipulator
Defeat the unborn raptor
Defeat the menace of the fertile river
Defeat the arsonist of the barren river
Defeat the corrupter of the cosmic river
Complete a green river without taking damage
Unlock all weapons
Unlock all companions
Find all health upgrades
Find all messages in a bottle
Get all achievements
Complete a blue river without taking damage
Complete a red river without taking damage
Complete a moonlit river without taking damage