I beat the final stage today with this team:
Usually I 4* challenge myself on legendary, but since I have done circuit of seasonal events I used some 5’s. This team beat every stage unchanged. Used Gretel and mana pots to make sure no fired. On some stages I let them fire if it was well in hand and irrelevant if they did or not. I typically will bring a second mana controller, but not this time. Wanted to play with my blue team. Gretel is my gal!
Guillinbursti and overheal made these easier too. I would time bombs to explode while his special was active increasing his damage out put. That helped clear boards and damage bosses of the board went dry on me.
I though Glenda’s minions might help more too, but their HP is low and by the time I was on advanced they seldom last more than 2 turns. 
. I guess it’s good I wasn’t relying on them.
I am a big fan of the event duration. It could be down at a much more casual and enjoyable pace.
Good luck to everyone else still working their way through.
