Sirene is a large area with a lot of branching paths, several collectibles, two optional bosses, and one story boss before you reach the final fight. Here is everything you need to get through it.
Entering Sirene
Walk along the main path until you reach a bridge. Cross it and you will see a platform with two purple flames on it. Walk toward the flames and a cutscene plays showing you the second Axon. When it ends, interact with the figure in the center of the platform. It works like a lift and takes you up to the next area.
Follow the main path from the lift until you find Expedition Flag: Dancing Classes on your right. Rest here.
Dancing Classes
Keep following the main path from the flag until you reach an open area where two ballet enemies are dancing. The path splits three ways here: left, right, and straight ahead. Go through each direction one at a time.
Right Path
Turn right and beat the enemy standing there straight away. Journal Expedition 55 is on the ground nearby so pick it up. Go around and you will also find a Colour of Lumina in this area.
Left Path
Head back to the center area and take the left path into the building. Inside, the path splits again. Go right first and destroy the Paint Spike you find there to get a Resplendent Chroma Catalyst.
Come back and take the left side path inside the building. Use the magic rope to go down to the lower level. At the bottom, you will see a grapple point ahead and a path turning right. Take the right turn first and follow it to find another Resplendent Chroma Catalyst and a ballet enemy. Beat it if you want, then go back to the grapple point.
Swing across using the grapple and follow the path on the other side to a paint cage. There are three locks to find and destroy. Once all three are broken, open the cage to get the Shield Affinity Pictos.
Step onto the Gestral platform to move to the next area. The path splits left and right here.
Take the left path first. This leads to an optional fight against the Chromatic Greatsword Cultist. It is a tough fight but worth it. Beating it gives you the Sireso weapon, two Resplendent Chroma Catalysts, and five Colours of Lumina. After the fight, look around the area to find the Effective Heal Pictos.
Go back and take the right path. There are ballet enemies along the way but you can skip them as they only give XP. Keep walking and you will find a Colour of Lumina on the path, then further on the Greater Shell Pictos.
Climb the structures ahead and crawl into the building at the top. Inside, pick up the Resplendent Chroma Catalyst. Find the balcony with a rope and use it to go down. At the bottom, turn right for another Resplendent Chroma Catalyst. After collecting it, follow the path back and it will bring you all the way back to the Dancing Classes expedition flag.
Straight Path
From the Dancing Classes flag, head back to the two dancing ballet enemies and this time go straight. A short walk ahead you will find a Resplendent Chroma Catalyst on the ground. Keep going until you reach another lift platform. Step on it to reach the next major area.
You arrive in a new area with a large statue in the center. Three paths branch off: left, right, and straight ahead.
Left Path from Statue
Go left. You will see a Petank platform and a Paint Spike nearby. Destroy the Paint Spike to get a Colour of Lumina. Leave the Petank for now and go back to the statue.
Straight Path from Statue
Walk straight ahead to a large dish. Collect the Chroma inside it. The Petank is nearby so you need to lead it back to the platform you found on the left path. Once it is on the platform, the fight starts. Beating the Petank gives you five Resplendent Chroma Catalysts, one Recoat, and five Colours of Lumina.
Right Path from Statue
Take the right path and follow the left direction when it splits. Beat the enemies here to get the Colim weapon for Lune. After the fight, follow the path further left to the very end to find a Colour of Lumina.
From there, drop down to the ground level. The Auto Regen Pictos is sitting next to a Benisseur enemy. Defeat the enemy to also get the Powerful On Shell Pictos.
Go back to the entry point of this room. On the other side of the room is a doorway. Go through it into a tunnel. When you come out the other side, two paths appear. The left one leads outside where a Resplendent Chroma Catalyst is waiting. The right one has the lift platform you need to continue the story. Grab the Catalyst first, then take the lift.
Walk forward a few steps and the path splits again. Take the right path and you will see a manor door ahead. Interact with it to go inside.
The Manor
You start in a bathroom this time. Open the door and shoot the sink mirror. A new opening appears. Go through it, drop down to collect the Shape of Health, then exit back to Sirene.
Back in Sirene
Follow the main path forward until you find a Fading Man NPC. Talk to him. After the conversation, look around the area and you will spot a Colour of Lumina in the distance. Go pick it up and continue into the next room.
Inside the next room, there is a grapple point straight ahead and an enemy on the right side. Go right first and beat the enemy. Follow that path all the way to the end to find the Protecting Death Pictos.
Come back to the grapple point and swing across. Follow the path on the other side to the very end to find a Colour of Lumina. From there, look for a pillar with another grapple point on it and swing across again.
On the other side, two paths open up. Take the left one first. Beat the enemy along the way and follow it to the very end to find a Resplendent Chroma Catalyst.
From where you picked it up, drop down to the level below. Go through the left door and a Mime is inside. Defeat it to get the Lune Haircut Sirène.
After the fight, go back to the area where you dropped down and head straight forward. Walk down the stairs and look for a slight left turn on the way down. Take it to reach Expedition Flag: Sewing Atelier. Rest here.
Sewing Atelier
There is an optional boss just past the archway ahead, but explore both side paths first before fighting it.
Take the right path from the flag and follow it until you get near a grapple point. A Resplendent Chroma Catalyst is sitting close to it. Pick it up and come back.
Take the left path now and climb the ledges until you reach the top. There is a paint cage up here with three locks to find and destroy. Once all three are broken, open the cage for a Revive Tint Shard. A Resplendent Chroma Catalyst is also along this path.
Optional Boss Fight: Tisseur
When you are ready, take on the optional Tisseur boss just past the archway. Defeating it gives you the Tissenum weapon and the Anti-Charm Pictos. If you want to skip it, use the grapple point to swing past and continue on the main path.
Past the boss area, look left for ledges to climb. Go up to the top and collect the Chroma there. Drop back down and follow the main path until you find a rope going down. Use it to descend to the area below. On the right side at the bottom, find Expedition Flag: Crumbling Path. Rest here.
Crumbling Path
Turn right from the flag and look behind the rubble nearby to find an Energy Tint Shard.
From the flag, head toward the edge of the area and take the rope down further. At the bottom, follow the path right to a grapple point. Swing across and follow the path to the very end where a crawl space is waiting. Go through it to find the Healing Fire Pictos on the other side.
Come back to the flag and head to the left side of the area. Find the large fallen pillar and look for wall structures behind it that you can climb. Take the rope down and a Petank is at the bottom. The platform you need to bring it to is just up some steps nearby. Before dealing with the Petank, check the broken vase on the left side of the bridge in this area for a Colour of Lumina.
Lead the Petank to its platform and beat it for five Resplendent Chroma Catalysts, one Recoat, and five Colours of Lumina.
Next to the Petank platform is a Gestral merchant named Klaudiso. He sells the Double Mark Pictos for 42,400 Chroma, the Energising Attack II Pictos for 37,100 Chroma, and Greater Powerful for 37,100 Chroma. You can also fight him. Defeating him in combat lets you buy the Chantenum weapon for 18,656 Chroma.
Boss Fight: Glissando
Go back to Expedition Flag: Crumbling Path and approach Glissando. This is a story boss so you have to fight it to move on.
Defeating Glissando gives you the Sirene Outfit for Lune and a Resplendent Chroma Catalyst. The path forward also opens up after the fight.
Head down to the lift platform in the area and take it to the temple area. Find the large column and pick up the Resplendent Chroma Catalyst near it. Also look for some boxes nearby as Journal Expedition 67 is close to them. Grab both and continue to Expedition Flag: Dancing Arena.
Dancing Arena: Final Boss
This is the last expedition flag before the final fight. Sort out your skills and attributes here before moving on.
When you are ready, walk toward Sirene. A cutscene plays as you approach and the fight starts right after.
Defeating Sirene gives you the Tisseron weapon, the Energising Turn Pictos, three Resplendent Chroma Catalysts, and a Recoat.
After the fight, a cutscene plays and you are taken back to camp. Your next destination from camp is the Monolith.