I see levels a bit lower than Tide does. I'm using PC defences average in the stat thread... they start at 9, they finish at 33. This is assumed to correspond to WA4 PC levels going from 1 to 50. It only matters for a few fights, perhaps, which I'll note. WA4 can, and frequently will, dump HP/MP to get abilities early, such as Slow Down (needs two GC spent) for Sinspawn Gui.
WA4 (Lv 12) vs Sinspawn Gui: Slow Down does its thing. FFX bosses are probably going to have a lot of trouble with this, because I agree with Snowfire about Slow Down hitting damn near everything (see also Spellbinding Eye, Charm, etc.).
WA4 (Lv 18) vs Spherimorph: lol Spherimorph.
WA4 (Lv 20) vs Seymour Guado (Macalania Edition. Please refer back to 'as literally to in-game as possible'.): First form is dangerous here potentially, due to guardians and their confuse. They also auto-potion away damage well and that kinda destroys WA4 since after Protect, nobody has enough damage to break it! But wait, Jude has ITEM STEAL. So uhh shremedies are still problems though. Confused Raquel does nasty things to her own team, and this is going to happen, with no real way to heal it? Yulie can try attacking if she wants to brave the evade... still eh, yeah, that probably does work since confused Raquel can't counter, but a missed de-confuse is bad. I'm not really sure about this. Assuming they conquer this phase, there's Anima to deal with, who now has a monstrous 7.5PCHP. Intrude chain hype is NOT working, it only barely works if you scale boss HP and we don't here. Still, they do go into this phase with 100 FP, which means two revivers. So... yeah, slow Anima, Yulie and Jude revive each other as need be, Raquel smites. Anima can still potentially grind the team's FP down here and certainly drains a bunch of their resources. Seymour, by contrast, is easier; his spells unfocus and all that. He can probably one-shot the fighters but can't do that much to the mages, so they may take him down with time... if they don't run out of MP. Still, this is rough. Resources are a serious concern here. Abstain.
WA4 (Lv 24) vs Evrae: As usual, only Rikku and Tidus can command the airship, so that's out. Anyway this is very, very tricky. Evrae has 2.7PCHP for his limit phase, so with 100 FP Raquel does barely kill that. Evrae will be using petrify a lot, which Arnaud can only heal at a big HP/MP hit (14 levels' worth), and Jude at a 25 FP cost. Running Arnaud out of MP is still pretty hard, so use it on Arnaud to force the FP down. In the meantime, Jude will attack three times, Raquel twice, and Yulie thrice or so? That's enough to generate the FP needed for Mystic. Yulie can also drop her HP/MP a ridiculous amount to cast Quicken, which helps certainly... probably enough to swing things, since she's barely not OHKOed this way. So, sure. Crazy the team manages this, I was convinced they'd have no chance. And the team is only just far enough in the game to buy Pygmalions (stone cure), even!
WA4 (Lv 32) vs Seymour Flux: Seymour can pick off Yulie right away. Arnaud however can slow Mortiorchis down and then Jude can revive once the needed FP is built up, same as with Evrae only Seymour is MUCH slower past his first turn. So that strategy is out. There's Dispel but the team won't be buffing. There's Cross Cleave but it fails to OHKO Yulie. There's Total Annihilation, but that only OHKOs Jude if Protect is on the others (and it will be, with that charge time). Yep, think WA4 has this. Mystic, everyone, Mystic! edit: Oh wait, slow counters. Okay, leave the mortiorchis alone, just slow down Seymour proper. Still, that makes things really hairy, since Mortiorchis can fire off Cross Cleaves every so often. Hum. I... don't think that makes the difference? The team just has too much healing/revival and gains oodles of FP (Cross Cleave has the annoying side effect of giving out 16). But it's close.
WA4 (Lv 38) vs Spectral Keeper: In theory, Berserk Tail turn 1 to stop slow/dispel, then Glyph Mine away two PCs... depending on the turn order, possibly even both revivers! Fortunately Berserk is only 50%, so usually this won't happen and slow down shenanigans are go. SK is still going to try to keep Arnaud berserked, though, and glyph mines at the wrong time are still pretty bad. Spectral Keeper can't be poisoned (and FFX bosses immune poison about as often as WA4 bosses), not sure about that hype. In general, I think SK will take this at some point, since the battle is long (8.7PCHP to cut through, Raquel ain't ITD yet) and there are too many chances for something to go wrong. I'd flip my vote if I saw anything from mystic curing berserk, but I don't.
WA4 (Lv 38) vs Yunalesca: Big, big problem with Snowfire's analysis: Yunalesca's counters dispel Hyper, she doesn't need turns. And Yunalesca has 6.8PCHP to cut through there at the end. Raquel's damage doesn't really get crazy good like it is in the DL (still good, mind, but we're talking 60-65% to average, not 80%), and even 1 hyper + 4 non-hyper physicals (needing one to kill the final form, but getting one from her own FP gains) aren't good enough here.
WA4 (Lv 50) vs Braska's Final Aeon: Yeah this is so much easier than the last few fights. How like in-game! Raquel has ITD now, petrify blocking is more available, Jecht doesn't have nearly the needed HP.
It's worth noting that if you see FFX slow immunity covering slow down, team WA4 is so fucked. (Don't worry, they still beat Spherimorph at least!)