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