How to get rid of cockroaches?
@Mazura is right... you will NEVER get rid of the cockroaches in an apartment.  The best you can hope for is to control them and minimize their presence.  

Some years ago, a friend was in your situation and she was unable to move out (the ONLY solution).  So, she packed every nook and cranny she could find with powdered boric acid.  That actually did an amazing job of controlling the problem.  

Indeed, the EPA publishes a document that says to blow the powder into the cracks with a hair dryer, and then caulk over.

http://www.epa.gov/ncer/childrenscenters/pdf/pesticidespring07.pdf

You should fully expect, however, that you'll run across some from time to time.