Buton's level in this fight scales with Rance's level and the current turn count, so you may or may not have screwed that fight over by powerlevelling the guy like that.
That said, have El ready with her Action Kopos, Reset (or Sill) with her S healing and All-Barrier units (Miracle/Parsley).
If you defeated Ssulal in a Solo Battle, you should have the Uruncel Blade, have it on Rance.
With this setup, if Rance is on a high enough Archfiend Tier, his solo Archfiend Charge will cancel any charges Buton throws on you, thus no need for Ninjas.
Essentially the strat (I use) is to use El to maintain your Shielders' Actions, use Reset/Sill to maintain Rance's actions, and have your one/two shielders cover your party at all times. Malicia is a great addition if you have her in order to maintain your Actions for a while longer if you're struggling with that.
Other strats include building up Bezeleye's passive and have her be the main damage dealer after bulking enough bonus damage. In that case replace Rance with Bezeleye (or whoever you merged her with), and Reset with a Ninja (with Uruncel equipped to cancel potential Charged attacks).
I had this exact setup, but Rance with uruncel blade just doesn't deal enough damage and Buton heals back up, only heavy attacks are enough to deal more than she heals back. I'm at around level 800+turn 72, all 10 in stats, archfiend level 12. I can't be overleveled, right?
So in my case I have Chaos, Reset, El, Miracle, Parsley, Rance. If I swap one of the all barrier units for a single barrier unit, Buton uses a row attack and I can only shield one character before she acts again... Chaos can't act before Rance, so Rance wastes half of his damage to remove shields, and there's always a moment where even with S speed, miracle and parsley just can't act before Buton, so I can only use Rance and Reset before Buton attacks again and then everyone dies. I'm not sure how you did it, I can't manage to keep everyone shielded at all times and deal damage at the same time. Also... playing on easiest difficulty.
Example with a different roster and uruncel sword. I can only save Rance. From then on I'm screwed because I'd need an army of monks and ying yangs lol.
I'm wondering how much extra grinding I'd need if I always used Bezeleye with uruncel blade every turn from the start, without doing any more dungeons than there are in the game (except the one with 999 floors), and using monks to give her extra moves in dungeons. By using her as main damage dealer, I could maybe let Rance be as low level as possible to weaken Buton lol.
Edit: Somehow I won through a bug. Buton killed my characters, they got replaced, buton disappeared, win. Lol.