The last boss is in a dungeon called Trial of the spheres. Yes it is hard. yes you should go in with someone. And yes, it has so many boss creatures in there that I cant even count anymore! :)
What is the last boss of Celestia? Is it a dungeon? Is it hard?
The final dungeon contains not just one boss. This is The Trial of Spheres, and it is hard. I could never find anyone to help, so went in and soloed it (with henchmen). It took well over 4 hours. Good luck! http://www.wizard101central.com/wiki/Quest:Trial_of_the_Spheres