Maribel's last form is when I busted out World Shaking Military Rule on Mari and Nitori, and then gunned her down with my best nukes in sequence.
IMO: Maribel's Last Form < Rinnosuke's First/Last
Hyperdimension Flying Object rips Tenshi to ribbons unless she's fully buffed and has around 250 wind affinity. Yes, it hits that hard, and for me, it killed her in one go at around 50% buffs, full buffs killed even Meiling (who took more damage, but sponged it with her health). However, with some creative gear (AKA, load up on the wind and spirit affinities for everyone who's going to eat a hit and debuff/paralysis/silence resistances for healers and stay-in attackers), smart usage of your own sources of paralysis and debuffs, her second and last forms go down fairly easy, as long as she doesn't Djinn Storm -> ONP -> HFO on you in that order.
Rinnosuke's Iai Slash is evil, especially after a full buff from WSMR. It can easily break through Tenshi or Meiling's massive defenses after buffs, and Start of Heavenly Demise after WSMR can deal some insane damage, especially if your buffs are running low. Oh, and Rasetsu Fist makes me rage, as it's a guaranteed kill to anyone with less than 6k current health, and a potential kill with less than 6500ish. And he kept sniping Tenshi with it.