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