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Moonlight Lullaby is the only reasonable option for 
Lucia, but 2-piece is flex.
Yunkui Tales is a good option if missing in substats to hit the 24,000 HP needed to max out Darkbreaker, otherwise damage sets like 
Woodpecker Electro, 
Hormone Punk or 
Chaotic Metal can help squeeze in a little extra damage.
If you're struggling to use EX Special every 25 seconds or having issues with making up daze thresholds, the 
Swing Jazz can be a good alternative for earlier (or extra) uses of 
EX Special Attack: Symphony of the Reaper - Daybreak.
Even with 
Dreamlit Hearth, it can be difficult having enough substats to hit 24,000 HP, so a full HP % set is needed. If, however, you find that you have more than enough substats (which can be easier with Signature and Yunkui Tales), then using a D4 Crit Rate or D6 Energy Regen is a way to get extra damage and daze respectively.
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4 - HP % | 5 - HP % | 6 - HP % |
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Yunkui Tales is tailor made for 
Yixuan, and there is no alternative that comes close to it.
For 2-piece, Woodpecker Electro is useful to make up for Crit Rate, but other damaging sets are fine too. As Lucia can run 
Unfettered Game Ball instead of her Signature, it may lead to overcapping on Crit Rate, so using 
Branch & Blade Song would be the best option in such a scenario.
As Pinnacle does not saturate any stats that can be built with via Drive Discs, the standard stat spread is ideal. Even still, the difference between Ether DMG Bonus and HP % is quite low, so whatever has more substats would pull out ahead. The same can apply for Crit DMG, although to a lesser extent.
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4 - Crit DMG | 5 - Ether DMG Bonus | 6 - HP % |
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King of the Summit's 4-piece effect is very strong and the only workable option, as an 
Astral Voice + Moonlight Lullaby alternative would only ever work with 
Astra Yao.
For 2-piece, 
Shockstar Disco is needed here. Daze thresholds are hard enough to meet with 
Ju Fufu as it is - even with an Impact D6 it can still feel tight at times, so it's the best option for consistent routing.
Crit Rate is required to meet the 50% bonus threshold from King of the Summit, ATK % is to maximise the value of 
Core Passive: Fu-rocious Might and Impact to meet Stun thresholds.
Running an ATK % D6 can work and it would be the better option, but this would be reliant on good luck with Miasma Priest attacks, as otherwise it may not be possible to hit 4 stuns without 
Roaring Fur-nace.
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4 - Crit Rate | 5 - ATK % | 6 - Impact |
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Miasma Priest specifically can have some tight daze requirements, and Lucia's complete lack of daze in her Aftershocks can be problematic at times.
She can, however, make this up with her EX Special Attack: Symphony of the Reaper - Daybreak and 
Basic Attack: Orbital Combo finisher shortcut. If required, try to save up enough Energy for after the 1st stun to sneak in an 
EX Special or 2 before and after the parriable onslaught of slashes.
The same concept applies to the Miasma Shield phase, where spending Energy actively can help build-up daze to rush through to stun (with no excuses to avoid, as not spending will often lead to overcapping due to the Miasma Purify Energy bonus).
This team plays very similarly to the Yixuan | Ju Fufu | 
Pan Yinhu or Astra Yao variants. There is very little change that comes from introducing Lucia into the mix, but that doesn't mean there is no change.
As Lucia has very few ways of activating Quick Assists, this can make opening stuns with 
Ultimate: Qingming Skyshade difficult. Yixuan's Basic Attacks will trigger Chains and holding Basic or Special will lead to EX Special Attack: Cloud-Shaper and EX Special Attack: Ink Manifestation respectively.
With Lucia's EX Special Attack: Symphony of the Reaper - Daybreak having its Heavy Attack on the very first slash and not the finishing hit means it builds no real daze before triggering a Chain. So combos with the comp are far easier to execute with a EX Special Attack: Cloud-Shaper into Ultimate: Endless Talisman Suppression opener instead, although making use of Ju Fufu to initiate the stun with her hold Basic or Hu Wei's off-field attacks can work instead.
On the flipside, Lucia's Decibel Generation is very good. This means it's possible to use Ultimate: Charge, Great Armor! mid-stun for extra Adrenaline recovery from Additional Ability: Auric Ink Undercurrent. On a fight like Miasmic Priest, it's possible to have it ready for every stun, making routing a lot easier and far less strict on Adrenaline management.
If struggling with daze build, using EX Special Attack: Symphony of the Reaper - Daybreak into Basic Attack is a strong option. It does come at the cost of some Dream Points, but maintaining Dream State should not be a problem.
Remember to keep using Lucia's EX every 25 seconds to upkeep Moonlight Lullaby, but aside from that, on-field Yixuan and following standard rotation practices will be enough.
