I have tried repeatedly to get past this fight but whenever the mysterious stranger casts the Upgrade spell the game stops and the only thing I am able to do is quit out of Hearthstone. Posting this so it is known as I cannot find any similar threads with this issue.
Any help or workaround solutions appreciated. Thanks.
edit - So I just restarted the game after a couple hours and surprisingly it let me reconnect to the last fight past where it had casted the spell. I was able to continue and finish the game after that.