Here is the full list of all 24 Inertia achievements.
Play a community created level
Take the harder route on Annex: Level #3
Complete all of the 'Unify' levels
Take the shortcut route on Unify: Level #3
Complete all of the 'Annex' levels
Begin the Unify Collection
Complete all of the 'Classics' levels
Complete 30 levels
Complete 25 levels
Complete 20 levels
Successfully use the zipline in an official level
Travel through a teleporter in an official level
Complete 15 levels
Complete 10 levels
Begin the Annex Collection
Save a level created with Creator
Enter Inertia's built-in level designer
Successfully wallrun in an official level
Completed 5 levels
Use a launchpad in an official level
Complete first level
Reach 1250u/s by bunnyhopping in an official level
Enter the playground/tutorial level
Successfully perform a slide in an official level