Scion Nanna

Weapon: Dazzling Palm Staff+ / Dazzling Staff of Twelve+ / Dazzling Observant Staff+ / Pain+ / Gravity+ (All can be Wrathful if Dazzling is used in B slot)

Assist: Rescue+ / Physic+ / Recover+ / Return+ / Restore+

Special: Miracle / Flexible

Passive A: Atk/Spd Push 4 / Atk/Spd Catch 4 / Atk/Spd Solo 

Passive B: Wrathful Staff / Dazzling Staff

Passive C: Spd/Res Rein / Atk/Res Rein / Atk/Spd Rein / Atk/Res Menace / Atk/Spd Menace / Joint Drive Atk / Joint Drive Spd

Sacred Seal: Blade Session / Atk/Spd Solo / Chill Res / Chill Spd

• Nanna's extremely high Speed and buildable Attack achieves a nice starting point for an all-out offensive build. As such, the usual "offensive healer build" skills apply; you'll be choosing from a staff of your choice and either refine it to be Wrathful or Dazzling, depending on the opposite Staff skill you're giving Nanna in the B slot. The combination of these two effects fixes Nanna's otherwise halved staff damage and, more importantly, grants her uncounterable attacks if the enemy doesn't have some form of Null C-Disrupt. 
• An all-out offensive build, of course, requires stacking any decent Atk/Spd skill investment you can offer for Nanna. 
  • In the A slot, Atk/Spd Push 4 is easy to use — Nanna instantly gets +7 Atk/Spd if she keeps herself above 25% HP — while being pretty common by five star exclusive standards. Atk/Spd Catch and Solo also provide similar effects with mostly doable conditions. 
  • In the Sacred Seal, another Atk/Spd booster like Atk/Spd Solo or Blade Session further stacks on Nanna's damage and follow-up potential. 
    • If those are occupied by other units, however, Chill Res or Spd are good substitutions that automatically apply a chosen penalty on a unit with the highest respective stat. They also work pretty well with Atk/Spd Catch from the A-slot, as one of the skill's conditions requires the enemy to suffer from some sort of penalty. 
  • Nanna's flier perks fire off greatly here as she gains access to the famed Rein line — thus, she would vastly prefer a Rein skill as her C slot passive as they basically apply a non-conditional stat swing of 4 in her favor. Spd/Res Rein in particular provides the golden combo of improved follow-up and damage, but any of the non-Defense-based Reins will do just fine. 
    • The Joint Drives are a viable substitution with a slight positioning caveat and ability to easily provide stat stacking for other allies.
    • A Menace skill (Atk/Res and Atk/Spd) is also just as great, potentially providing an effective visible stat swing of a whopping 12 (+6 on Nanna and -6 on the enemy) if you position Nanna and her enemy just right. Like the Chill skills, the enemy penalty also plays right into the debuff condition of Atk/Spd Catch.
• Lacking a unique staff, Nanna has to choose between some inheritables.
  • While keeping Nanna's default Staff of Twelve is mostly fine as a useful passive support option, it doesn't provide stat swings necessary to maximize combat potential. The same applies to staffs with otherwise useful post-combat effects like Pain and Gravity. 
  • To maximize combat potency, you'll be pretty much limited to looking at Palm Staff or Observant Staff (at this time of writing). 
    • Palm Staff is easy to get off a Heroic Grail-obtainable unit and essentially inflicts Atk/Spd Rein on the enemy, granting better follow-up potential and overall survivability. 
    • Observant is exclusive to a once-a-year groom unit and has a slightly strict and specific positioning caveat (Nanna must be three spaces from two ally-supported units), making its overall worth a bit questionable. Unlike other staffs, though, it features a technically superior payoff of all stats (especially Attack) +6 in combat while cancelling enemy bonuses. 
• Nanna's healing Assist and Special are mostly inconsequential given how rare the higher-investment ones are and the build's focus on attacking. If you can provide a heal with good positioning effects (Return or Rescue allows Nanna to rescue allies from enemy range, while Physic heals at a safe distance), that would be ideal but far from necessary compared to the other skills. Likewise, you can just plop the common Miracle on the Special slot and call it a day (assuming lack of preferences or otherwise good inheritables).

Weapon: Dazzling Staff of Twelve+ / Wrathful Staff of Twelve+ / Dazzling Candlelight+

Assist: Recover+ / Restore+ / Physic+

Special: Heavenly Light / Miracle

Passive A: Spd/Res Form / Atk/Spd Bond / Atk/Res Bond / Atk/Res Solo

Passive B: Wings of Mercy / Chill Res / Chill Spd / Aerobatics

Passive C: Distant Guard / Atk Tactic / Drive Atk  / Other Tactics, "Within Two Spaces" skills, etc

Sacred Seal: Spd Tactic / Distant Guard / Other Tactics, "Within Two Spaces" skills, etc

• It's easy to deride Nanna's pretty lacking base skillset outside of her nice default Staff of Twelve. Ironically enough, though, Atk Tactic (an easy method of supporting allies in different movement classes), Heavenly Light (for lack of great healer Specials in the low rarity pool), Recover+ (an additional +10 HP is always good regardless of base healing amount), and Spd/Res Form (a cheap way to improve a healer's follow-up avoidance and bulk) are skills that are typically inherited on low-cost, support-only healer builds. If that's all you foresee as far as investment is concerned, all you have to do is fill in the B slot and Sacred Seal — Wings of Mercy or some Chill of your choice are cheap choices for the former, while adding Spd Tactic (or some other one of your choice) to go along with Atk Tactic doubles up on visible buffing. 
• Of course, if you're in a mood to dress up a little more finely, you have several low to mid-cost options for doing so, especially if you're planning on using Nanna outside of general gameplay or being an Arena Assault backup.
  • Instead of Recover, Restore or Physic are slightly preferred choices due to their respective secondary benefits — penalty erasure or safer distant healing. 
  • As usual, Miracle is a no-thoughts-given, situational but useful (when it is) choice if you foresee Nanna taking many actions and charging it fully throughout the match. If Nanna sees combat, it's a handy survivability option.  
  • For a more offensive twist on the build, you can run an Atk-based A passive along with the Wrathful refine of Staff of Twelve, giving Nanna some reasonable bite if she must enter combat.
  • Rather than run Tactics, Nanna can run a variety of in-combat or movement support skills in her C Passive and Sacred Seal slots, which is far preferred for Aether Raids Offense of Defense play. For instance, she can stack "within two spaces" passives such as Distant Guard or Drive Atk, or utilize her flier class to help other allies move via Guidance.
  • For a dedicated Aether Raids Defense setup, Aerobatics helps Nanna cover more ground with other non-flier allies around her, and Flash provides a legitimately threatening option against unprepared Offense teams without Null C-Disrupt. Nanna should be paired with a Far Save armored ally to make full usage out of these skills.