Had the same bug with my friend yesterday, at the first boss. Here's a temporary solution that might help people until this gets fixed:
We downloaded the save game editor from here:
http://steamcommunity.com/app/239070/discussions/0/648816742499208285/ (try to PM me if download doesn't work anymore)
We converted the save file (in your hammerwatch folder in /save) into an xml file and edited it right at the beginning:
<string name="level-id">boss_1</string>
to
<string name="level-id">3</string>
and put ourselves back into floor 3 where I could rebuy all my skills. After entering the boss level, we backuped the save game (because afterwards the bug occured almost evertime we wiped at the boss). I'd recommend to take backups of your savegame after that on a regular basis.
Might be that this doesn't work on another boss. At the end of the savegame file I found this:
<dictionary name="spawn-pos">
<float name="x">-24</float>
<float name="y">20.25</float>
</dictionary>
Which might be the player spawning position. So when you spawn somewhere stupid, I'd say edit these coordinates.