Developing Fianna: Secure healing first, then add Speed and durability for magical swordsMajor spoilers
Use mounted healing for a regular build. With heavy single-route investment, gain Defense through class changes; across routes, develop magical sword damage alongside healing.
Original video: The strongest healer? The strongest Swordmaster! Fianna development guide | Bilibili creator Qiongyuan Destiny.
The healing weaknesses behind high Magic
Fianna has excellent Magic growth, but her personal abilities and spell list favor healing rather than fully exploiting it offensively. As a healer, she also faces low Speed, poor physical durability, and no ranged healing. Those are the cards available; the task is to play them well.
For low investment, keep her healing in the rear and use mounted movement later for positioning. With focused single-route investment, make extra class changes for Speed and Defense. To create a core unit who both fights and heals, use different growth paths across all four routes.
Build Luck before mounted healing on the regular path
Follow the 
Priest line in the first two parts, avoiding combat and developing Luck. Both healing output and sustain benefit from Luck. Reliable healing is more useful than forcing a slow mage into damage. Stay behind the team and avoid entering enemy range just to heal someone.
In Part 3, use 
Valkyrium. She must approach allies to heal, making mobility important. Low Movement leaves her behind the front or trapped when enemies arrive from the rear. Mounted movement helps her keep up and choose positions. Her spell uses and abilities already suffice, so a regular healer can remain here without occupying a Divine slot.
Healing Staff improves healing, 
Silver Shield adds physical durability, and personal recovery items fill the remaining space. Keep a way to save her as well as others. Equip 
Black-Magic Mastery, 
White-Magic Mastery, 
Gift of Faith+, 
Barrier, and 
Piety. Piety adds 3 Luck, while Barrier adds Resistance beside allies, helping wear down enemy spell uses.
Prioritize 
Canto on the mount. After approaching to heal, moving back two tiles reduces front-line risk. A Luck-focused mount suits her, but the ability matters more. I obtained Canto on a Ferghanan Horse and used it for that ability.
For intensive single-route training, raise Defense before Intercept
For serious development on one route, start on 
Leda's, where she joins early. Instead of staying with Priest, use 
Shaman and 
Ovate for better Speed and Magic growth while finishing enemies and breaking tough targets. Once healing uses and output cover her job, improve other areas.
Train armor and enter an armored class once. This is for minimum stat modifiers, rather than frontline tanking. If Defense is below the class floor, changing classes raises it. After returning to magic, the class's own bonus disappears but the base increase remains. Repeating the change does not grant Defense every time.
In Part 3, first use 
Castle Knight for Defense and mastery of 
Intercept, then return to Valkyrium. Intercept reduces incoming damage by 5 against both physical and magical attacks. For further investment, enter a Divine class for 
Divine Law and 10 Magic, then free the slot after mastery.
This improves Speed, Defense, and room for error over the regular healing path. Keep equipment light and use 
Gale Ring for Attack Speed. First avoid follow-ups from fast enemies, then aim to double slow ones. Gift of Faith+ supplies healing, Divine Law and mastery abilities add damage, and Intercept supports survival, making her a secondary core fighter.
Use different routes to address different weaknesses
On 
Cai's route, train 
Charioteer with a Ferghanan Horse for HP, Defense, and Dexterity. This supplies physical foundations; leave Speed to another route.
On Leda's route, her early arrival allows 
Hunter, 
Rogue, and 
Ranger for Speed. Focus level-up rerolls only on Speed. On 
Dietrich's route, use black magic, going from Shaman to Ovate for Magic. She joins late on 
Theodora's route, so follow Priest classes for Resistance and Luck. Develop each strength separately and combine the results.
Start Part 3 as 
Shadow Seeker for more Speed. Once Divine classes unlock, master 
The Blade and finish in 
The Avatar. The goal is a well-rounded unit who deals sword damage and heals.
Leave enough Attack Speed in the magical sword setup
Main damage comes from 
Levin Sword and 
Salamander, with 
Cleansing Blade for physical attacks when opposing mages resist magic. Use black magic as appropriate, and heal or rescue with white magic when no good attack is available. With physical foundations developed, carrying a sword is no longer merely decorative.
Do not fill every slot blindly. Whether to carry Silver Shield and Healing Staff depends on the remaining Attack Speed. Keep their utility if Speed stays sufficient; go lighter if they slow her too much.
Use Speed +10 and Divine Law, adding 
Swordfaire and Black-Magic Mastery for offense and accuracy. Gift of Faith+ maintains healing. If healing is already sufficient, an extra 
Wing Soldier change can provide 
Lightning Reflexes. Favor battalions with Dexterity, Defense, and Charm to balance other bonuses.