We have just made public a new version of the PC version of Boss Defiance, build 1.0.4.This patch will be applied by Steam automatically when you next start the game.
The patch solves some issues with the achievements:

Bug & Fix

• Achievements were not working properly and were not unlocked: from now on you should be able to unlock all the achievements.

Should you experience any difficulties or If you find any bug, please report it here or at info@tinypixel-studios.com.

Thank you and have fun with Boss Defiance!!