Guide
How the kit works
Bring Hydro into the fight and use Countermeasure: Lumi's Battle Cry! to send Lumi against nearby enemies. Her Super Power Form deals AoE Geo DMG. If Moondrifts are nearby, Lumi also hits nearby enemies with damage considered Lunar-Crystallize Reaction DMG.
After using the skill, repeatedly tap the skill or Normal Attack to feed Lumi. Once full, she attacks in Ultimate Power Form with a powerful AoE hit considered Lunar-Crystallize Reaction DMG, then switches to Standard Power Form. Tapping the skill also increases Linnea's interruption resistance.
Use Memo: Survival Guide in Extreme Conditions when the party needs healing. It summons Lumi in Super Power Form and heals nearby party members, then continuously heals nearby active party members for a short time based on Linnea's DEF. If Lumi is already present, the Burst resets her duration without changing her form.
Capture Protocol (NA) has a three-shot Normal Attack and a fully charged Geo Aimed Shot. You don't need to rely on charged shots for the Lumi sequence. Outside combat, Linnea's quick arrows can help collect forging ore or capture certain animals, subject to a weekly capture limit.
Build and why
Use four pieces of Aubade of Morningstar and Moon for Lunar Reaction damage. It gives 80 Elemental Mastery from two pieces. The four-piece effect increases Lunar Reaction DMG by 20% while Linnea is off-field, with another 40% at Ascendant Gleam. The effect disappears after she has been active for 3 seconds.
Husk of Opulent Dreams is another option: two pieces give 30% DEF, and its four-piece stacks provide DEF and Geo DMG Bonus. For team Elemental Mastery, try Silken Moon's Serenade. Its four-piece effect grants the party 60 or 120 Elemental Mastery at Nascent Gleam or Ascendant Gleam after Linnea deals Elemental DMG, including from off-field. Its Gleaming Moon effect also contributes to Lunar Reaction DMG.
Choose DEF Percentage on the sands and goblet, with CRIT Rate or CRIT DMG on the circlet. For substats, look for CRIT Rate, CRIT DMG, DEF Percentage and then Elemental Mastery. DEF helps both the Burst healing and Linnea's Lunar-Crystallize passive.
Golden Frostbound Oath is the first weapon choice. It gives 16% DEF and, after an Elemental Skill or Lunar-Crystallize hit, increases the wielder's Geo DMG and Lunar-Crystallize Reaction DMG by 40% for 6 seconds. With nearby Moondrifts during that effect, other nearby party members also gain 20% Geo DMG. Aqua Simulacra is next, with 20% wielder DMG near opponents even off-field. Elegy for the End offers 60 Elemental Mastery and can gain its Sigils from skill or Burst hits off-field.
Leveling priority
I suggest leveling Countermeasure: Lumi's Battle Cry! first, since Lumi's attacks are a large part of the rotation. Follow with Memo: Survival Guide in Extreme Conditions for healing, then Capture Protocol (NA). Tapping Normal Attack feeds Lumi, but ordinary bow attacks are not the focus here.
For maximum ascension, gather 420,000 Mora, 168 Etherwing Moth, 1 Prithiva Topaz Sliver, 9 Prithiva Topaz Fragment, 9 Prithiva Topaz Chunk, 6 Prithiva Topaz Gemstone and 46 Plume of the Fallen Watcher. You also need 18 Tattered Warrant, 30 Immaculate Warrant and 36 Frost-Etched Warrant.
To take all skills to their maximum, you need 4,957,500 Mora, 3 Crown of Insight, 9 Teachings of Vagrancy, 63 Guide to Vagrancy, 114 Philosophies of Vagrancy and 18 Elixir of the Heretic. The skill total also includes 18 Tattered Warrant, 66 Immaculate Warrant and 93 Frost-Etched Warrant.
Constellations worth it
Tidings of Joy and Sorrow is worth considering for a Hydro and Geo team. For 8 seconds after Moondrift Harmony, it gives those party members 40% CRIT DMG. Lumi's Ultimate Power Form hit also gains 150% CRIT DMG. At Ascendant Gleam, the specified Lumi attacks trigger Moondrift Harmony and count the party's Hydro and Geo characters as having applied their elements.
Provisional Classification adds Field Catalog stacks when Linnea uses her skill or triggers Moondrift Harmony. Lunar-Crystallize damage from nearby party members consumes stacks for an increase based on 75% of her DEF. Lumi's Ultimate Power Form hit can consume up to five stacks for an increase of 150% of DEF per stack.
Golden Beagle's Dream improves that earlier effect: the skill or Moondrift Harmony grants maximum stacks, and consuming stacks gives a larger damage increase. At Ascendant Gleam, nearby party members' Lunar-Crystallize Reaction DMG is elevated by 25%. If you are leaving constellations aside, the base Lumi and healing loop works without Eventful Log Page, Expert Instinct or Fairyland's Farewell Gift; those add skill levels, DEF and Burst levels respectively.
Teams
Linnea converts Hydro Crystallize triggered by a party member into Lunar-Crystallize. Every 100 DEF she has increases its Base DMG by 0.7%, up to 14%. Lumi also reduces nearby enemies' Geo RES by 15%; at Ascendant Gleam, the reduction is another 15% after Lumi is summoned.
Common mistakes
Don't leave Hydro out if you want Linnea's Lunar-Crystallize conversion. Also keep the fight near Moondrifts when possible: Lumi has an extra reaction-damage hit when they are nearby, and Golden Frostbound Oath has a teammate effect tied to them.
Don't switch away immediately after summoning Lumi if you want her Ultimate Power Form hit. Repeatedly tap the skill or Normal Attack first. On the other hand, with Aubade of Morningstar and Moon, remember that its four-piece effect disappears after Linnea has been active for 3 seconds.
Don't build solely for healing and forget DEF's other uses. It also raises Lunar-Crystallize Base DMG and determines Linnea's Elemental Mastery grant. When using the Burst with Lumi already present, remember that it resets her duration rather than changing her current form.
