@Michael
What's worked for me is this. (As usual, you should back up your site and database prior to making such changes)
Copy the files from the 3.4.2 package into your wordpress installation folder on your server. However, skip the following: wp-content folder and xmlrpc.php file.
Once the 3.5 files have been overridden with with 3.4.2 files, just log into your backend and Wordpress will ask if you want to update your database. Hit yes, and you should be back to 3.4.2.
I don't know if this is the best or perfect method, but it's worked for me so far. Good luck!
