Thanks for your reply ; Indeed, as I was trying one more time (this time with the hammer in case that would have been important in order to win), I noticed the "Preparing spell" (or something like that) inscription on the screen so I disrupted and was able (thanks to the firepower I had brought... catapults + mages) to kill him quickly after that.
So that was not a bug. Sorry for the faulse alert.
Being my first time in this forum, I'm not sure if there's something to do to close the thread but if someone can do that please do.