I beat Celestia at around level 60-61 And oh my goodness as a Character it was quite challenging. I did take quire a few amount of breaks so of course i am not very experienced when it comes to using Myth characters. But they are indeed fun!
When I finished Celestia, I was around level 58. This was because I hadn't done many side quests in the previous worlds, which now I realize was kinda a mistake. Now that I am done, and have to be a level 60 to enter Zafaria, and now I have to do a bunch of side quests from Celestia to get there. But, good luck with the rest of Celestia! :)