skipped a few levels, beat 24, then the (-) won't let you go back to beat the others.
Also, the walkthrough at http://jayisgames.com/archives/2011/07/impasse.php#walkthrough works, if you find the comment "Posted by: DAM Author Profile Page | July 13, 2011 2:56 AM"
Hey you can get special badges along with the unlocks. I just got one for completing 4 of the objective/unlock things in one level. Try "Killer" mode and avoid the target zombies to really level things up. + If this helped you!