Is Stellarys good against Luminel?

The fight speeds up basic attacks, and almost everything Stellarys does runs through basic attacks. The buff lands on the part of its kit that already does most of the work.

Its basic attacks restore energy, so the loop is to basic attack until EP is full, then spend it on skills. Using a skill triggers its trait, Grand Sorcerer, which adds damage to its basic attacks for 5 seconds. Faster basic attacks mean EP refills sooner, and you land more hits inside each 5 second window. The Shadow Magic clone copies its basic attacks and skills at 20% of its normal damage, so it gets more swings out of the buff as well.

Staying alive against Alpha Luminel

Stellarys is a ranged Dark damage dealer, so you can keep hitting from a distance instead of trading blows up close. Its Glide skills, such as Shooting Star Glide, ride a star through the air to reposition and dodge attacks while dropping stars on your target. Using Shadow Magic again swaps you with the clone, which gives you a second way out of a bad spot.

What to hold

Fission Needle is the best held item for Stellarys, Prismana form included. It boosts basic attacks, so it stacks with the fight's speed buff rather than competing with it. The full build is covered in the linked Stellarys build article.

Not settled: The exact effect of Fission Needle is not settled. It appears both as a 30% increase to basic attack damage amplification and as a 50% chance for every basic attack to deal double damage, so how much it adds in this fight depends on which one is right.