If you've followed our last guide for Hades II, which told you the best way to take down Hecate, you'll have found your way to the game's next area. Oceanus is full of annoying enemies. Even the guy who can give you some buffs can be irritating, too, but the trio blocking your exit from the watery world are the baddest of the bunch.
Scylla and her Siren band are a visual and audio treat when you first encounter them, even if they are a deadly one, too. Unlike Hecate and the later bosses, who tie directly to your mission trying to take down Chronos, Scylla is just in the wrong place, at the wrong time, but rather than let you pass, she sees it fit to try and force you to listen to her band's set, which usually involves getting blasted by plenty of attacks.
All three members of the band share a large health bar at the top, but you can't just focus one to bring them all down. When each of them has taken enough damage in the first phase, they'll stop attacking but also become invulnerable, and then in the second phase they'll start to perish one by one. When you first get into this fight, you'll probably be tempted to go for the drummer first. While this is a valid strategy, as the drummer is immobile, you'd be best staying on your toes, as she can blast you with AoE damage, and her fellow musicians aren't just going to let you wail on her. If you've got some area-of-effect of your own, it would be best to start the fight by rushing right into the middle and getting that damage in while Scylla and her drummer are close together.
If you want to avoid the drummer's attacks and can't time dashes properly yet, your best bet is to run to the far side of the map, where you can kite the guitar player. She can lunge forward to strike you or hit you with a series of projectiles that knock you back. She's more of an annoyance than a big damage dealer, but if you don't watch out for her diving attacks, you might end up falling victim to them.
Then there's Scylla. The real boss of the bunch, she moves slowly but is completely invulnerable to damage aimed at her back. She has a screech attack that lasts a very long time and catches in a wide area, she can shoot a bubble of music at you, and can clamp down her clam shell to shoot a multitude of spiked projectiles across the arena.
Once you've done your prep, picked your weapon, gathered boons and equipped whatever keepsake you think will give you the best chance of survival, the best way to approach this fight is to think about the Sirens as three individual enemies you have to take down. Usually, leave Scylla for last as she is slow and can be dodged easily. The drummer is the simplest foe to attack first, as she can't move and her arena-wide attacks can be relentless if left alone. The guitar player is an annoyance, but she doesn't have many attacks and so if you can get her alone, she's really not that intimidating.
Scylla on her own can be more than a handful, though, and so we'd also recommend on your journey you pick up something from Hestia, Hephaestus, or Zeus. Hestia's scorch effects mean you can dodge into melee and out while leaving the foes with some lasting burns, and Zeus and Hephaestus offer great AoE options if you can drag all the band members together. Also, the Selene Nightmare Form is incredibly useful here, as it gives you some much needed seconds of invulnerability.
In short, there's not much we can say to give you an easy win here, except play smart rather than fast. It's easy to get overwhelmed with three bosses coming at you, but don't be afraid to get some distance and plink away with ranged tools if you have them. Otherwise, just kill the drummer and you should be onto a winner.