If I were to narrow it down to just one boss, it would have to be Estrakir Gloomthorn.But overall I like any boss that is a minion of Morganthe (Shadow-Weavers, Goliath, Etc.)
Sure you have a favorite boss! Come on, share it to the Wizard101 Community!
I was totally amazed by the Jade Oni in Mooshu, he was also the first fight I did that I ran out of cards and that was back when I put them all in my deck and my sprite minion was the only one left throwing her small 2 pip spells at him, got him down to under 500 and couldn't take it any more and left for the next go at him which also failed but the third time was a charm but I did have help that time, him being life and putting up shields for my death converts did put me at a disadvantage.
