|The purpose of this build is to let Lyn peel the enemies, so something else can finish the enemies off. As she is a staff user, she can also fill the spot in your party as a healer. It works well against every type of enemy, as they cannot counter her no matter what and have their movement reduced to the minimum of 1, leaving Lyn safe with a single Distant Def 3 or Wrathful Staff 3 from an ally.
She fits best in a mixed party, as you can have an armor to heal, a flier for mobility and kiting or a cavalry to finish the wounded enemies off.
Weapon:'"`UNIQ--item-249--QINU`"' is used to reduce all enemy movement to armor movement and prevent them from attacking next turn with help from a friendly assist. With the refinement it also adds good damage to her hits.
Assist: Optimal is a great option for the healing skill, since after they updated the healing to scale with Atk, she will be able to recover 30+ hp. The reason I didn't choose Rehabilitate+ is because I prefer keeping my units at high health instead of recovering almost dying units, but that is personal preference really, so you could choose what you prefer.
Special: '"`UNIQ--item-255--QINU`"' is the special Lyn comes with, and can be kept without any worries. If the build is used correctly, she will never need to heal anyone.
A Passive: Gravity+ is used because there aren't many offensive A skill options for a staff unit. You could consider using Atk/Spd 2 to help a bit with the damage, but since it is harder to get and doesn't help that much, Speed +3 is more recommended.
B Passive: '"`UNIQ--item-261--QINU`"' to prevent targets from countering Lyn's attacks.
C Passive & Sacred Seal: '"`UNIQ--item-264--QINU`"': is used to cause collateral damage. It stacks with itself and Pain+ so it does 14 damage in collateral damage. That's a lot of damage.