Leanne

This is an all or nothing defensive build for Leanne focusing on her strengths. She is purely a support unit and should rarely be used for fighting. If you're interested in making her an offensive unit thats able to attack, then I suggest going somewhere else because that's not what we're doing here.

So here's what were dealing with. When it comes to her stat line it's very clear to see Leanne is going to struggle against any physical units and will probably shatter like glass in a single hit. Her atk is poor and her def is downright atrocious. However, her spd and res are easily some of the highest stats we've seem on dancers to date which means she won't be getting doubled any time soon. This is incredibly useful when on flier or beast teams because as long she has access to buffs she'll almost never be doubled and her defense may reach high enough to survive a single attack from unbuffed low to mid tier atk stat enemies. They shouldn't be getting to her anyway with her movement but things happen.

When it comes to her special anything can be put here. She'll almost never use it so it doesn't matter. I personally drop miracle here for runs through tempest trial because you can at least abuse the mages and clerics there with her to charge it.

Her A slot is interchangeable with her seal slot most of the time so it's easier to get these two on her depending on what you've got to work with. I use darting blow simply because on the minimal chance she survives an encounter she shouldn't be hit again if she's going to make it. Other skills are acceptable like steady posture, close/distant defense, fortress skills, etc though this is what i recommend. On the chance a ploy build is being used, the optimal option is Fortress Res/Def 3 or Fortress Res 3 to ensure ploys are going through.

Her B slot is dependent on the team she's with. A team of dragons or beasts will normally give her enough movement she won't need flier formation often while a team of fliers is gonna be better off with her having access to it since she won't transform near as much. Other options include renewal if you feel like that's necessary, desperstion but it's extremely circumstantial, or any of the chill skills which is what i recommend. Alternatively, if you'll have her tanking hits from mages mystic boost is a great inheritable choice in order to mitigate any damage received.

Her C slot functions best with a team buff skill like hone or ward and besides. There's really no other reason not to take these anyway. On the other hand she makes a great ploy skill user so this is an alternate option.

I take warding blow 3 into her seal slot but again, anything to help mitigate damage is preferred. If she is being used as a ploy unit, Fortress Res 3 is your best option.

Refreshers can be very handy, but I typically disliked using them. I often found them a burden as their offensive and/or defensive stats tend to be lacking(with some exceptions, of which I do not have), sometimes enough to be nigh impossible to patch up with skills. I finally found one I like, in Leanne. I decided to slap together a mixed support/offensive build onto her and see how it goes.

Her Asset/Flaw is important, as she desperately needs +Atk/-Def. Her Def is terrible even if it is an Asset, and her Atk is barely serviceable when it IS an Asset, and we need all the help we can get.

Her Prf is basically the only weapon you can justifiably use, as alternate Beast weapons aren't really a thing, at least not at the time of typing this. Thankfully it is a great one, giving her +3 Spd(that's a total of 40 at base!), a much-needed +2 Atk and +1 movement if she transforms at turn start, and providing a 7 HP heal to all nearby allies at turn start. A nice blend of offense and support, the inspiration of my build.

Obviously we keep Sing on her. No reason to change it.

Moonbow is the only thing that will proc often enough to be worth a damn. Luna is fine if you're willing to accept a 3-count. Normally I wouldn't mind, but she' frail and I can't afford to have her sit around attacking normally when a Spec might be just what she needs to hang in there.

So yeah, Life and Death 3 or Fury is obvious. They are clearly meant for characters who possess mediocre stats and need that extra push to make them viable, though of course it comes at a risk. Thankfully, if you opt for L&D, Leanne's Def is guaranteed to be atrocious anyway, so dropping it to 7 is not really going to change anything, and her Res will dip to 29, still a respectable amount, especially if she gets an added Ward, or Fortify from an ally. Fury 3(or 4) can give her a nice boost in all stats, but with the risk of her taking damage(We can make use of this!). Death Blow 3 or 4 is another option, for obvious reasons.

I was scratching my head for a long time, deciding what to do with her B slot. I decided on Desperation as it is both accessible and useful, and pairs well with Fury. Vantage would normally be better for a melee unit, however, even with Life and Death 3 or Fury 4, her Atk won't be high enough to OHKO most units. Desperation, provided she survives long enough to use it, is preferred as it makes up for her Atk only being decent with this build. Dance skills, or Wings of Mercy might be preferred if you want to lean a bit more towards a Support role. Flier Formation/Guidance might be viable too, depending on team composition.

I decided Hone Beasts is fine as I plan to put together a Beast team someday(Leanne and Nailah are currently the only Beasts I have...I'm hoping for Velouria, and Lethe to fill the remaining spots), and it works as a great support skill. If she isn't in a full Beast or Flying team, skills like Def Smoke, Drives, Tactics, and Waves will prove useful.

I opted for the Res Tactic seal as I figured the rest of the team can appreciate a Res boost, especially Nailah, her DC enemy Phase teammate. It can also be handy for a Beast team, as most have middling/poor Res. Def Tactic is viable too, of course, but I currently don't have it as of writing this. Anything to aid her offensive or support potential is helpful. Go with whatever works best for you.

Ultimately, Leanne is never going to be a truly effective attacker due to her status as a Refresher with below-average Atk. However, her Atk is potentially good enough to be boosted to respectable levels with the right build. She also begs to be a support unit, and thankfully, mixing the two, allowing her to be decent in both roles, is quite possible. Just, be careful. She's about as resilient as a petroleum-drenched paper bag in a house fire.

IVs: +Atk/+Spd - Res/-Def

Skills
A: Life or death 3/Swift sparrow 2/AtkSpd Bond
B: Flier formation or movement/dance skill
C: Goad Fliers/Beasts
Seal: Any skill that improves atk/spd or movement skill (flier formation if not in B).
Special: Moonbow/Glimmer

The point of this build is that Leanne has great offensive stats that if improved a little with skills became amazing OHK. In exchange she will die with one hit most of the times too.

The B skill is preferred to be his natural Flier formation, that combined with her weapon allows her to position perfectly for killing enemies or dance a teammate and stay safe.

In all three slots A, C and seal should be aimed to improve speed the most attack second.

With glimmer and moonbow she will trigger special every time she doubles an enemy, and that will probably be always happening, if pored with the right team.

Also team buffs will turn her into an amazing nuke.

Best boons for Leanne would be either +Spd or +Res and best bane for her is -Def.

If you want Leanne to be tanky, do a double atk/def bond because 'during combat' skills stack and if she's next to someone, she'll get +10 attack and defense and if she's next to her ally support, she'll get +12 attack and defense. Some other good bond skills that involve atk and/or defense are other good options or you can keep her default skill to tank dragons in enemy phase

Wings of mercy is the best skill for any dancer/singer because they can reach anyone under half health and it's also good to troll people in AR and throw them off.

Hone beasts is good on beast emblem teams. Other good ones are hone/fortify fliers for flier emblem and guidance is good in non-flier/beast emblem.

Iceberg is the best special for her because of her high res. Moonbow is good too but it won't do a lot of damage as iceberg. Glacies is also good but it'll take a while to proc

I think the best IV Leanne is +RES/-DEF
This Leanne (+RES, -HP) is a Magic Counter on Enemy Phase, and a classic support singer during the player phase.
Obviously, Distant counter on slot A is the heart of this set.
I've put Windsweep on slot B because Leanne takes part of my defending team. With Windsweep, she can't be counter-attacked by ennemies which target her very poor defense. She's a little more safe during player phase managed by CPU.
C-slot depends a lot of her crew: beats buffs, fliers buffs or some Tactics tricks, if she works in a mixed team, could be interesting.
This seal HP/RES II rises her magical Bulk.
This set need some buffs for helping Leanne to be really effective. A RES buff (giving by a teammate) is important to control more surely the most dangerous magical attacks, like Ward Flier or RES Tactik III.