if you think you found a bug:
1) create a new thread on the support forum
link:
https://forums.taleworlds.com/index.php/board,376.0.html
2) include the description of what happened
3) post a link to download your savegame (you can find it on your documents folder if you have Windows).
--> publish it on Dropbox, etc and get a downloable link (public)
3.1) If you have a backup with the quest active before the fight, include that one as well
4) post your computer information: OS (Windows, Mac or Linux), ....
5) post your rgl_log.txt (you can find it on the same folder the game is installed)
6) Are you using any tweaks or mods?