Specifically how do you deal with those 900k attacks that Grand Magus throws out? It's not all of her attacks so it's not like her attack is just that high, there's some sort of gimmick that I just can't figure out. Also they seem to have oddly high avoid, I'm missing like a third of my attacks even with Kesia's blessing giving +300% hit rate.
Edit: Beat it, though I got lucky and Luna didn't actually use one of those 900k damage attacks and Molly used Auto-Life on L'ightning instead of Luna. That being said, if you have sufficient damage output on one dps and use Lyra's surprise attack blessing (or equip first strike on everyone I guess?) you can use immortality on that dps to kill everyone and just keep some ailment negation item/blessing on them to avoid getting slept. Ribbon's negate basically every status effect of relevance so just slap one of those in a blessing slot.
Worth noting that I was using Scarlett's blessing to make damage shared between all enemies, it might have actually been a better move to instead focus fire Luna down instead. I also used the blessings from all the Monster Mashes and Ladder, both of those are easier to fully complete than the Ultimate Tournament and they give you neat blessings to boot. I also did my typical move of using Dante's timewarp and lots of blessing swaps to inflict every DoT debuff possible on them but it honestly might've been better to just stick to dealing damage.
I've attached my character setups for reference, one of them should be carrying Lyra's blessing all the way up to the final battle and then you swap it out for something useful immediately.
