Right then. First a quick reply to Jo'ou...
I dunno, you -do- have to consider that Purim's buffs are getting neutered by the MT (each extra target that is aimed by a Purim MT spell lowers its effectiveness and duration by a fair amount. Spread around five PCs, the boost just sorta fails), Garnet's buffs have awful awful duration and can't be MT'd, and I'm still having a hard time swallowing any DQ8 buff not named Acceleratle or Oomph doing anything but fail horribly. Turns spent buffing are also turns spent not making any offensive leeway - which is not good in a bevy of fights that's heavy on strong MT offense.
* Jo'ou Ranbu nods. Then, OK just forgot to note it. Does figure, though, Insulatle already blows being MT, it'd just be worthless if ST.
DQ8: Eh, the DL is a place where random spells and abilities you didn't use are suddenly and randomly awesome. And it's not like Insulate is even bad in game, just rather specific when it's good - it's fantastic in the boss fight in the Dragon's Graveyard, for example, changing a reasonably tough fight into a joke.
SoM: I don't believe that's correct? To my knowledge, Defender and other buffs have reduced duration when cast MT, sure, but their power is the same. And I don't want to get into an interpretation holy war, but just say that I don't agree with punishing a cast in a way that's impossible to happen in the game anyway by scaling the power down more past 3, and also note that spell effects are not consistent in how they scale. Lucid Barrier, for example, is at 50% strength when cast MT, not 33%... it's impossible to say how it would scale in a 4 or 5 target situation. Oh well.
The debate: Actually, by my count, I'm losing 4-6 right now? Maybe I counted wrong, which I wouldn't complain about, but even if it the vote was more lopsided I'd want to write up my thoughts.
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Next, resources... Jessica has basically an infinite bag of MP (~250 MP with a Whip). She regenerates MP with her Staff and Kazing /Kaboom cost a mere 15 MP. Kazing is full revival, too. Everything else is even cheaper - TDL / Accelerate / Insulate are 3 MP and everything else of note is near that. Assuming she never ever uses her staff that's 15+ Kazings, and the early fights are likely fought with a Staff out so the MP cost is only coming from the extra map MP+100 mana.
Rena's pretty excellent as well. She can theoretically run out, but if she does then Garnet surely has, so I'll skip her.
Purim I posted the spell list on earlier, but she's got 55 MP or so and most of her spells cost 2 MP. She doesn't want to spam Revifier - more of an emergency backup - but that's okay. Revifier also brings back the character with 75% health, as a note, if something better than Garnet's 50% health is needed.
Garnet... Cura's ST max healing, or MT decent healing (60%ish?). Curaga is MT max healing. Life's 10 MP, Cura's 10 MP, Curaga's 22 MP, Shell's 6 MP. As for her summons... assuming you allow the unique Odin's Sword skill, she's doing her best damage with Odin for 28 MP (and it's actually pretty excellent, checking the stat topic - 8125 damage at endgame). If you don't, then Bahamut does cost a bunch of MP at 56 MP... but Ifrit is only 26 MP, and it still does about 75% of the damage of Bahamut. Garnet isn't summoning often anyway, and the difference should only come into play when someone's HP is right on the edge of mattering - IOWs, in the Dungeon, Ifrit is just fine most of the time. She's got maybe ~280-300 MP.
Okay, more specifically, to focus on what Garnet & Purim are up to... I'll try and take into account multiple interpretations here rather than just my own.
Team Snowfire vs. Reno, Rude, Elena, Fujin and Raijin
Garnet uses a Summon, Purim uses MT Blaze Wall. This fight does not seriously last into round 2. Garnet's summon is even theoretically optional here, the team is pretty owned by Firefly.
Team Snowfire vs. Rofel, Adramelk and Kletian
Garnet uses a Summon, Purim uses MT Blaze Wall or Lucent Beam. Actually... now that I think about it, especially for those who hold to LOL FFT boss HP, this fight is very likely over in round 1. FFT average speed is 10, same as the bosses.... except that Worker 8 / Beowulf / Reis / Cloud all really shouldn't count, and all of them except for Cloud are below 10 speed (they're 7/8/9/10). Meaning that the party average speed is likely a nudge above 10, meaning Purim gets the drop on the enemy team, and now they're eating Kaboom / Garnet Summon / Purim spells before even going along with a Lyn double. (Or, if you do say they're all average, tiebreak for Purim due to her being a PC.) I guess Kletian likely survives thanks to his Mantle, but he's not getting anything interesting done. I suppose the point is, even if you respect FFT HP enough to let Rofel / Kletian get to Round 2, Garnet likely doesn't need to summon round 2 to mop up.
Best case: Garnet 1 Summon, Purim 1 Attack spell
Bad case: Garnet 1 Summon + Life / other healing (?), Purim 2 Attack spells
Worst case: Garnet 2 Summons + healing during mop-up round 3, Purim 2 Attack Spells + Cure Water
Team Snowfire vs. Rubicante and Barbariccia
Assuming FF4 original:
Rubicant is dying early round 2 for sure (the whole Ice Sabre Jessica, Twin Dragon Lash thing). Val & Rubicant can combine to kill Lyn round 1 and Weak someone else at worst I guess? Rena is resurrecting Lyn, though, and Garnet Curas the Weaked person. Now Val is stuck messing with Lyn, while Lyn gets revived. If you see magic as able to hit Val, then it's not a big deal as Purim / Jessica quickly zap her out. If you don't, then it's still not a big deal due to the accuracy buff that Purim has on Jessica.
* Standard case: Garnet ST healing (2 Curas?), Purim Ice Sabre + Blaze Wall (1x Blaze Wall + 2 Kabooms should do it for Val)
* Val can dodge magic case: Garnet healing (4 Curas & a Life? Val isn't MT, no need for Curaga), Purim Ice Sabre + Speed Up + (Cure Water's Light Healing?)
Assuming FF4 DS:
If the team wants to waste my resources rather than go for an MT magic kill, Barb never spins and just attacks Lyn. Keeping up with Inferno, if you let Rubicante spam it, requires either an MT Curaga from Garnet or Cure Water + Fairy Light. The fight is over as soon as Barb misses Lyn as Purim then does the Moon Energy -> Lyn kills Rubicant thing. If Barb stays lucky, then Jess Accelerates on turn 2, so double turns start kicking in; Rubicante shouldn't last TOO long.
* Garnet Shell x1-2 + Curaga x1-4 (heal Inferno 'till Rubicante's dead), Purim Cure Water x2-4 + Moon Energy (Heal Barb's attacks on Lyn, make Rubicante explode)
The numbers vary as it depends on when Lyn evades / when Accelerate based double turns start kicking in.
Team Snowfire vs. Xenobia and Royce
Obviously a bit of an interp split here. Also impossible to tell which boss is faster in initiative, as they're both slower than Dragon Cane Mia. But plenty of Stone healing here (Garnet's Stona, Purim's Remedy, Rena's Dispel all work) and furthermore Defender is turning off their chances of OHKOing someone assuming you let it do anything at all. Presumably they have to both target the same person to get a proper KO going. If you respect their Stone, their Stone is not consistent, and Jessica also has Magic Barrier. It reduces statuses that would normally hit 100% to hitting 50% of the time, statuses that would normally hit 85% of the time to hitting 20% of the time, and statuses that would hit 50% of the time to hitting 10% of the time. There's no way their stone chances are better than 85% (stat topic doesn't say how good the odds actually are), so I'd consider 20% a good cap on their potential of hitting with it - meaning it really sucks and is nigh-worthless aside from making Jess spend a turn to Magic Barrier. It's also attached to a physical, meaning that I'd argue that if a physical got entirely nulled due to Lucid Barrier, it has no chance of hitting anyway.
Anyway, as noted before, I really think that the ladies are in big trouble here. It's an easy win if Flamebird doesn't turn off Firefly, but assuming it does they're still being massively outsped and running into a metric ton of buffs. Due to different interpretations, hard to guess what people see going on here, but even if for some reason Jessica can't start spamming Ice Sabre / Growthed Twin Dragon Lashes, Purim can still kill Royce with Lucent Beams. Once Royce is down, the fight's easy. Admittedly, this fight IS where the resource issue really comes into play, if you see the two as consistently killing Jessica and thus forcing Purim to do the job.
* Garnet 2-5x Cura, 2-5x Life (?), 0-2x Stona, 0-2x Shiva on extra turns. If they go for MT damage Cura not being full is fine, Rena can cover the rest. Life only matters if they go ST on Jessica. Purim: Defender, 1x Ice Sabre or 3x Lucent Beam, 0-2x Cure Water (emergency healing?), 0-4x Lucid Barrier (if you respect Stone?). Note that this is too much, she won't be casting all this as which set she casts depends on respect and the strategy Xenobia & Royce pick.
Team Snowfire vs. Zog, Melfice and Zera Valmar
Once Zera goes down, this team has no shot. Melfice is cool but he's not really an HP god (great for Grandia II, admittedly, but his competition sucks), and the pressure is off my team once Popey dies thanks to the hyped buffs (Anti & Defender, really, messing with Melfice / Zog's magic and Melfice's Demon Horde Slash). Melfice shouldn't be able to take that much punishment? Zog admittedly can take a ton, but by then he's rammed into Anti + Insulate and is doing utter crap for damage.
* Garnet: 1-2x Curaga (heal Mel's initial Demon Horde Slash, perhaps toss another in for safety on round 2), hang out. She can toss in Curas, but others can handle the healing once the two are under control; by now their damage is extra bad, and Rena can start healing once Anti is cast anyway. Purim: Defender, Moon Energy so Lyn kills Melfice quickly, and whatever (2-4x Cure Water to take up the slack if Garnet wants to take a break from healing?).
END RESULTS:
Best case: Garnet is casting 2-3 Summons, 3 Curagas, 1-3 Lifes, 4-8 Curas, and some cheap utility spells. This should take ~200 MP, well within her bounds.
Purim is casting 2-3 Defender, 0-3x Lucid Barrier, 2x Arbitrary attack spells (probably Blaze Wall as it's cheaper than Lucent Beam), 2-10x Cure Water, some random cheap utility spells (Remedy is 1 MP, Speed up is 2 MP, etc.). This should take ~35 MP, easily within her limits and allowing for random emergeny revivals with Revifier.
Worst Case: Garnet is casting 3-4 Summons, 4-5 Curagas, 5-6 Lifes, 6-10 Curas, and some cheap utility spells. This should take ~330 MP, which is a bit too much.
Purim is casting 2-3 Defender, 3x Lucent Beam, 0-3x Lucid Barrier, 4x Arbitrary attack spells (probably Blaze Wall), 6-12x Cure Water, some random cheap utility spells. This takes ~60 MP thanks to the damn Lucent Beams, also a bit too much; a reduction of only 1 puts this back to safety.
In general, I lean toward the "best case," obviously. If you see everything as going against me, then yes, the team can run on out of MP... but eh, all I can say is that I don't see that happening. And it only takes a few things to not go as badly wrong as I describe to push the MP amount back to doable - this isn't "get through the dungeon with plenty of MP left," it's "get through even if the gas tank is sputtering at the very end."
Obviously, your results may vary, so adjust as you see fit based on fights you don't agree with my take on.