ShinUltima I've made a few attempts at Slydra now and I've gotten close a few times with this:
Rosa - USB2
Celes - BSB2/USB/Arcane
Squall - Ice Glint/BSB2/USB2
Laguna - imperil USB/Chain
Aerith - USB2
Everyone is dived (I don't want to talk about diving Rosa...). Celes/Squall/Laguna have HP/ATK/RES waters applied, Aerith HP water, nothing on Rosa. Very low magia on everyone, this actually got me to set up an auto clicker which I'm doing now to address that. I can probably clear this without but no reason not to spend a day or two or whatever auto'ing 200 levels for mainstat/HP to make it easier. Squall/Celes/Laguna both have two +ice pieces equipped, and that ice artifact sword is maxed.
On my few attempts I got close, but mostly failed due to just not timing my last stand heals in phase 3 correctly, or I should have popped Celes' Arcane and didn't, or I got super unlucky with Icy Offering not imperiling etc. I could also probably stand to tweak RM's; I might still cap or close to cap with Squall to drop a w-cast spellblade RM on him or something maybe. I've got Celes' enIce LMR but she caps almost immediately so I think she's better off with the +SB gauge against weakness and w-cast LMRs instead. I brought her BSB2 because one time I ran out of SSS hones (it's r5) but it only happened the one time, I feel like generally the fight's gonna be over one way or the other by then. I feel like going glint into BSB2 with Squall is making a good difference over just running the BSB2; I haven't tried running his USB2 (the one with 4-8 hit chase) instead his BSB2, it's probably more consistent but I'd also have to hone my 2nd SSS from r3 to r5 to run it and I feel like it's kind of a wash. Laguna does Icy Offering into Chain, then casts his imperil USB with extra bars when available. Aerith USB2 is a no brainer, and I also have her AoE heal LMR which is great.
I'm running a 2nd healer because Slydra's DPS is still nuts, I don't know that running an entrustbot or something really does me any better? Also magia might help but I have to run DMT on one of the two healers, I can't survive quite long enough to get them up normally. I tried using Elarra first instead of Rosa running bard stuff between USB casts but once Chain is up I'm capping so it felt kind of useless?
I'm pretty sure I can clear it with this team (and magia will help), but I"m certainly open to suggestions. My only other potential dark horse option is crazy, but I've got a fully decked out Noctis with AASB. Ice is *not* one of the elements that triggers his w-cast and he has no enelement, but with chain up and his 100% crit etc I bet the 6* ice samurai ability would break cap for 5 hits which might help with savage in phase 3, but I feel like that's realllllly a stretch. Rosa definitely feels a bit awkward on the team, I've got most/all of the other top healer options, something like Deuce or Relm would also be good but I think the shortcharge I'm getting from Rosa is overall gonna do me more good?