Luna 5/6.4 V1 Stygian Onslaught writeup from beta testing, with impressions from Uncle L4UR (can't fit in this post), for Fearless difficulty.

🍄 Mandragora
The initial single target phase lasts 35s if you don’t use Pyro or Electro. The AoE phase lasts 17s, though it can obviously end sooner if you kill all the spores.
There are 10 spores, you need to kill at least 8 to make it lose its resistances in the following single target phase.
Kazuha can group all the spores with just one Skill, they seem to be less spread out than the overworld version when they spawn.

🦆 Knuckle Duckle
The fight plays the same as the abyss version, when you down it the first time it stays down for 15s and will redo the shield phase around 40-45s since losing the first shield.
The ducks are meltable/vapeable. They have a lot of resistance to everything except Electro, it's 150% in Abyss, I'm not sure if they increased it and by how much for Stygian, would need a dataminer to be sure.
When the ducks are killed, they don't deal a fixed amount of damage to the boss, instead some of the damage dealt to them is transfered to the boss, I'm not exactly sure what the conversion rate is for now though.
The amount of hits from EC/LEC to break the ducks has increased, it seems to be at least 12 or slightly more, will try to do a more thorough test soon.
The ducks aren't groupable, but they also don't get knocked around by Overload.
Each duck has 3.3mil HP in Fearless, 6.7mil in Dire.