This Ex Dungeon is not too difficult, with the right team, but otherwise the damage dished out is quite high. You'll want either a mono-water team, or as close to it as you can get. Note, this dungeon does not allow use of gems to continue.

The fire element boss, Shusui, hits incredibly hard, but thankfully doesn't have much other tricks up his sleeve. There's just the main buff removal wipe and ultimate attack every three turns. If you guard all units on every third turn, and be prepared to heal next turn and you'll be fine. Also bring status curing, he does curse.

Mono-Water Team Recommended

For a mono-water team, you'll want a mix of an HP 30% leader (Selena/Kurt) and 15% Fire reduction LS. (Sergio/Zerafaga). Elimo is the best water mitigator, especially since her combination heal/mitigate/status-removal is really good for this Ex Dungeon. Even though you probably have Selena for healing, depending on your setup, you might be forced to use her normal attacks most if the time to generate BC. If you only have Oulu, I would not recommend him for this, he's deadweight (no BC generated, no damage, no buffs) and low HP. Unless the rest of your team is exceptionally strong (Taser, Kurt, Selena etc.) then maybe. But you'll have an easier time using just about anyone else in a mixed team (see below).

Leaders: Selena or Kurt; AND Zerafaga (best) or Sergio. Selena/Selena will also work passably well, but it's not the best combination of Leader Skills imho. Kurt/Selena will work also. Selena/Taser, can work for some, but you're really lowering your safety margin without that second LS to lower damage or up HP. If you're bringing along any 6 stars, you'll want Selena/Kurt or Selena/Zerafaga.

Good sub units from best to worst: Selena (if not leader), Taser, Kurt, Medina (new water unit); then Kuhla, Eve, Rigness, Karl, Belfura, Arius, Noah, and dead last, Sergio (if not leader). Sergio dies fairly easily, just a warning. I wouldn't use Sergio except as leader here. (Noah note: if using Noah, be careful not to use Noah's SBB with 25% mitigation to override your normal 50% mitigation, remember last buff is the one that sticks. Or use his BB only)

I would only recommend mono-water if you have Elimo, Selena plus either Kurt or Taser. If you can do this setup, the dungeon will be very easy, almost autobattle territory. If you can't, then go with a mixed team instead.

Spheres: Drevas and MagicOre/EmpressTear or better on Elimo. Lexida/TeraGenerator on Selena, or at least a Sol Creator. Legwand/Malice on everyone else.

Partial Water Team Also Recommended

The next best setup would be a partial water team. Since you're not bringing mono-water here, you'll really need both 30% HP LS and 15% Fire Damage Reduction. Kurt works best here, with 30% HP for all, and increased crit damage, plus he deals high water damage. And to pair with him, again 15% fire damage reduction (Zerafaga, Sergio and Granados). But pack in as many good high HP water units as you can. If more than 2 units are not water element, you might want to bring a water buffer. Generally, you'll want to make the highest damage and best BC producing team that you can, the fewer turns you need to kill him, the fewer of his ultimate attacks you'll have to survive.

Leaders: Kurt and Zerafaga/Sergio/Granados. Zerafaga is the best by far. Sergio is good. Granados is ok. Kurt alternatives: any 30% HP LS. I'd suggest Grah, Griel or Kanon. Griel would be preferred. Oguro (with Melchio sphere and Seria Ex sphere) works, plus he fills BC when attacked. Lastly, Emeldia is surprisingly good leader here, with 40% HP LS (plus small chance of recovering HP when attacked), can remove status and has HoT on SBB.

Fantastic sub units: Selena, Taser, Kurt, Medina. Great sub units: Feeva, Diana, Kikuri, Griel, Sefia (or any 7 star that increases BC fill that isn't earth element). Water unit filler: as before, Kuhla, Rigness, Karl, Belfura, Arius and Noah. Water Element buffers: best is Eve, followed by Keido (Quaid, whatever) with Edea/Melchio/PalmnyaGQ spheres. (not Loch's sphere, the 4 heroes sphere) or Lucca if no Keido/Eve. Decent sub units filler: Damage boosters; Raga, Ruby, Kira, and etc. any atk, crit or spark buffers who aren't earth and have high HP.

Mitigators: Elimo is still the best here, followed by Shera, Aaron and Darvanshel. Narza should also work, as long as you have sufficient BC generation. I would avoid using Edea unless she's your only mitigator. Even with her sphere that turns her into a neutral element, she'll still have fairly low HP. Lastly, if you're not going to be using Elimo with Drevas, then don't forget to bring someone else who can remove status.

This would be my general recommended team, because not everyone has Elimo. Or even if you do, you might lack good water units to go with her. This way, you at least get some of the benefits of having some water units. You may need to bring a revive or two, but it shouldn't be all too difficult.

Double HP Leaders Last Resort- not recommended

For sake of completeness, I wanted to mention that some have cleared this dungeon (close calls) with double HP leaders that were mixed element teams. I don't recommend it though, if you have any weaker 6 stars or low HP units, they'll be wiped out easily at his three turn ultimate attack. But hey, it's here for your reference.

Leaders: combinations of Keido/Selena/Kurt and: Grah/Kanon/Owen (the raid guy). Again, Edea/Melchio/PalmynaGQ sphere on earth units. Oguro might work too, haven't tried him. Mitigators: preferably Elimo, Aaron or at least Shera/Darv. Other units to bring: Taser, as always. Ivris for healing/status removal. Other units should increase BC/HC or just BC, the usual Feeva, Kikuri, Sefia, Griel, Diana etc. Do not bring Lilly Matah, she'll just die. With this setup, you'd absolutely avoid bringing any 6 stars that aren't Shera/Darvanshel/Narza or Rigness. (Although when I tried this myself Rigness died right away)

You'll need to heal from the red every single turn, and probably bring revives too. Not recommended except if it's your only choice. But if you have one HP 30% leader, then why not find a Zerafaga/Sergio friend? It will make things much simpler, and you won't need almost a full team of 7 stars that way. It is possible though, with some luck.

And, that's all! Don't forget to guard all every three turns! Enjoy your shiny sphere frog (and 1 gem).