Whether you've tried all of the old methods involving vsync and fullscreen and framerates or not, this method is something you should probably try if you still can't get it working. However, it does take some effort as well as a knowledge of the steam console, or at least how to open it.

Essentially, what you're doing is rolling back to an earlier version, as I've noticed a correlation between the beta being pushed forth into the live version and the impossible boss issue.

Step 1 - Open the Steam console by typing the link "steam://open/console" into your URL bar in whichever browser you use. (There are other methods, but this one should work most easily.)

Step 2 - Type "download_depot 205090 205091 7401943240205080825".

Step 3 - Wait. It won't give you any sort of progress bar, but it shouldn't take too long if you have a decent connection. DO NOT exit the steam client while it is downloading. When it finishes, it should tell you where it downloaded to.

Step 4 - Retrieve the entire folder and put it somehwere you won't forget about it. (You can even leave it there. Just make sure you know where it is for the next step.)

Step 5 - Go to the directory into which the game is installed. Copy everything you just downloaded from the folder from the previous step into this folder.

Step 6 - You're done at this point, though it may update back for some reason if/when you restart Steam; if this happens, just repeat step 5.

Once all of this is done, you can play the game without fearing that the first boss will keep rallying your shots for all of eternity! I've also managed to confirm that acheivements do in fact work, and leaderboards seem to work as well.