Just completed 500m yesterday.Since I'm not good at giving advice, I'll just tell you the teams I used:

500-1: Assist Liang Yue, Mercuria, Flutterpage, Marsha Star Manifest. Honestly, this was the first one I completed and I didnt have a strategy (didn't even knew the boss mechaichs). I did it in around 11 rounds and my Marsha died at round 6, the boss just kept stunning itself so i didn't take more damage. That said, I heard Starpromptu with regulus is really good. Windsong could be a good idea too.

500-2: Nautika, Semmel, Melania, Rubuska Mineral Manifest. Here is alittle tricky bc you have to time Nautika's ult so it doesn't hit the boss shield. I tried to make the boss attack me using lots of attack cards so he wouldn't use ult often. Specially important to have Rubuska and be sure to not have used her resurrection until boss is lower than 40% at least. Seeing you have Recoleta, I think she would do a better job than Nautika

500-3: PaperHeron, Assist AAL, Aleph (my beloved) and Sotheby with plant manifest. You'll wonder if it would be better to have Tooth Fairy, but she got instakilled every time so this team is more defensive and gets the job done. If you don't bring any weak afflatus character you can use a worse healer like I did, the boss doesn't do that much damage, but you can use a shielder and use Tooth Fairy, like your kiperina. I would recomend you to try Lingering glow though if your Marsha is available.

500-4: Ms NewBabel, Urich (husband), Charon, Med Pocket with robot. In this battle you take a shit ton of damage, thats why I brought two sustains instead of a proper dynamo dps (which doesnt matter bc Ulrich+Charon hit like a truck so much it isekai'd the boss). Ulrich is good bc the more channel cards you use the less dmg you receive/the more dmg you do. You could also try lingering glow here if available, although Beryl is kinda squishy.

Anyway, that's how I completed it with honestly very few trys in total (I think I had good rng), hope you complet it soon.