Moved, thanks! 
I’ve been recording the Monster and Boss HP, which I’ll hopefully have time to type up at some point and compare to the notes here: 🌟 Memento Challenge Events - #3 by Mariamne
I’m working through Rare still, but it looks likely that it’ll be fine without Emblems. I’m through 12 so far.
EDIT: I’ve made it through all of Rare now, and would say it’s comfortably beatable with unemblemed 3* heroes if you have a good team, and use some minor items as needed. As usual for Challenge Events, mono works well.
I haven’t compared the Boss/Monster stats yet, but it anecdotally feels relatively comparable to current Challenge Events.
I do think it’ll be a bit harder to get competitive runs in, though. Completing is pretty easy, but doing so quickly will be hard in the higher Stages.
@yelnats_24 FYI