Credit to Cereal for his tip on double Laguna sphere on Relm.
First off, for this to work you need to slot 2 Laguna E- refined spheres on Relm. This allows Relm to apply mBRV down twice per Tentacle attack, which is enough to uptick the Lufenia orb.
Sylph summon allows Relm to heal Sephiroth the weakling in between the bosses recast attacks.
The fight just requires careful monitoring and refreshing of the Sketch Debuff which is key to Relm's damage.
Also do NOT use Seymour LDCA or any other CA that applies a generic mBRV down. If you do, Relm will not be able to uptick the Lufenia orb with her Laguna spheres anymore.
Timestamp
Party setup at 22:36
My Relm has a 2/5 UW.
I removed all speed passives on everyone to make them as slow as possible. On hindsight though, best to leave speed passives on Galuf and only remove on the other 2 members.
0:00 Battle start
Don't be like me and waste a Sketch Summon here. Best to save all of it for when you need to heal.