|The purpose of this build is to let Nanna 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 Nanna safe with a single Draw Back or Reposition 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: Gravity+ 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 prevents the enemy counter-attack which is important, as Nanna is pretty susceptible to death as her def isn't that high.
Assist: Recover+ 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: Heavenly Light is the special Nanna comes with, and can be kept without any worries. If the build is used correctly, she will never need to heal anyone.
A Passive: Speed +3 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: Wrathful Staff 3 to increase the damage on the target, not hit by Savage Blow. Alternately, Guard 3 works on a budget while denying the locked enemies the chance to charge their specials.
C Passive & Sacred Seal: Savage Blow 3: 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.