So, Celerity Nightmare down.
Edge L70 - Dismissal R4/Leg Shot R4 - Knight's Charge - Eblan Dual Wield
Zack L67 - Lifesiphon R4/Full Charge R4 - Battleforged - Air Strike
Zidane L65 - Silence Shell R4/Blind Shell R4 - Soaring Dreams - Shift Break
Thancred L69 - Paralyzing Shot R3/Tempo Flurry R5 - Ace Striker - Dancing Edge AND Death Blossom
Tyro L80 - Curaja R5/Halting Rumba R3 - Rebel's Might (Didn't know what to stick on him, so I went with this. Selphie's RM3 probably would've been better since I had him MND-specced for the Stop, though) - Freaking CELEBRATION GRIMOIRE.
RW: Shout
So, this one took a few tries. I'm tempted to Shout at the start of the fight, but instead I have everyone except Thancred and sometimes Tyro use their physical - Thancred uses Paralyzing Shot, and Tyro pops a Curaja if someone got hit. Eventually the first Tonberry goes down, though I have to mess with things a bit to make sure Tyro had a turn queued when this first one died.
Part 2 is when Tyro having a turn queued meant he could use Halting Rumba almost immediately. So I do that, and still am saving resources here, though I pop a Shout halfway through this wave.
Part 3 is when it gets dicey, but Tyro's opening Halting Rumba hit all three to start with, and then the next one hit at least two of them, so I managed to control the fight from there and still save most of my resources.
Part 4... Yeah, that's a damage race. Between Edge's leftover Dismissals and Zack's Air Strikes, the boss wasn't getting a lot of turns, and while Junk was an MT 2HKO to my entire party... Celebration Grimoire healed it all away. Twice. Thancred got the finishing blow with his last Tempo Flurry.
Lost one medal for actions and one for damage, but eh. Fight's done now, and I mastered it.
EDIT: As an after fight thought... I want to say that this is one of the hardest Nightmares so far. That or I was just really underleveled/underprepared for it.