"I pulled W.Fae/Eirika/Ephraim +this -that. What do I do?"

All three of these units are very powerful and have a lot of flexibility due to how many sheer stat points they have.

Fae: comes with unprecedented amounts of res, solid atk, and middle-of-the-road def for a tank. Her spd is a bit lower than I anticipated, and without a lot of investment she will get doubled by a lot of units. She can build quite easily to take the doubles.

My strongest recommendation for her is steady stance 4 because it brings her def up in line with her res and protects her from enemy specials during her enemy phase.

For a B-skill, the choice is mainly between bold or vengeful fighter. Wary fighter can be good in some very specific strategies (ex: on AR offense, trying to tank everything just to get an AR opponent into attacking mode so that they extend into a vulnerable position where the rest of your team can pick them apart). I like vengeful fighter on any build; bold fighter I like but less on lightning breath builds because the only one-round specials that activate with it are moonbow and glimmer. GamePress has suggested the old lightning breath + steady breath + bold fighter + QR combo, which is decent but with two main drawbacks. One is that she's often going to get doubled into a special, and potentially a huge one if her opponent is a bold fighter. Even if there's no huge special, there's a good chance her QR is busted after one round of combat. With other builds she can be much tankier. Examples of alternatives:
Lightning breath + steady stance 4 + vengeful fighter + bonfire/iceberg or blue flame because it's funny (NB bonfire/iceberg may actually hit harder if you exceed two wards, whether ward dragons or ward armour or some combination thereof). Note the use of a 3-charge special even when expecting to get doubled due to the cooldown penalty on lightning breath.
Lightning breath + steady stance 4 + vengeful fighter + aether. Same deal, but aether only activates every other round (or you can charge it up to fire on player phase.)
Lightning breath + steady stance 4 + bold fighter + moonbow/glimmer + QR seal
Light breath (glittering is OK too) + death blow + bold fighter + bonfire/iceberg + atk smoke seal (note that she has given up on enemy phase. She will not fight well on enemy phase with this build; she sets her teammates up to be able to do it well.)
Glittering breath + DC + bold fighter + QR seal + bonfire/iceberg. A double-edged setup with a higher rate of ORKO than the old breath skill + aether but with weaker sustain power.

IVs: all IVs are good; +HP is the least good. +atk, +def, and +res are all good, but if you're not using steady stance 4 then +def might be best. If you're using steady stance 4 with +def or +res then refine her weapon for whichever stat is not increased in the boon. I also like the idea of -spd to try to force the opponent to double her so that 4CD vengeful or 3CD QR specials will activate, but it can get her killed if the opponent has an anti-armour weapon, and smart axebreakers who plan to crack wary fighter probably have enough spd to double -spd W.Fae.

+spd is good with enough investment. With just a weapon refine and darting stance seal, she gets to 38 spd, which blocks a decent amount of doubles, and she can still go for more spd in the A-slot.

-HP: is probably preferred
-atk: make sure you get enough damage from your special. Might want to stay away from aether unless just surviving is enough for her role on the team. And don't forget that she hits the lower defensive stat of most characters - adaptive damage against ranged, since a lot of units won't use mystic boost, and res against melee, which is not a strong stat on at least 50% of melee units.
-spd: already discussed
-def: invest in her def. It's worth it. Probably start by refining her weapon for def. Steady stance 4 is good. Steady breath can be good. The list continues.
-res: can bring it back up with the weapon refine, especially if using steady stance 4, but if you're using fort. def/res or death blow or something like that you might just refine her weapon for def and balance her bulk that way.

Fodder options:
- very nice weapon, but Nowi and Fae will get prfs eventually and lightning breath is perfectly fine until then. The Kanas already have water breath, which functions extremely similarly, and is there another manakete who doesn't have a prf?
- Def/res bond is one of the better bond skills because bonds are relatively easy to utilize on enemy phase, but it does have a drawback in that it may expose Fae's bond partner to ranged fire, especially if the opponent has a dancer. If Veronica is involved and has savage blows equipped then the situation can get messy or even ugly very quickly. Def/res bond also has to compete with fortress def/res, which is more consistent at the cost of 2 atk and the risk of getting chilled. (Fort. def/res also helps ploying.)
- Vengeful fighter
- Armour march

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Ephraim: the most straightforward of the three. Due to the solo condition on his weapon, I strongly suggest keeping him on player phase. If you cannot inherit armour march to him, his teammate should provide it to him. Trying to play enemy phase using a solo skill forces teammates to stand further back from the action, inhibiting them from getting involved in the fray once it's their turn.

Atk/def solo is fine to keep; if you have an extra atk/res solo that can also be considered because his res is actually decent, especially when the solo condition is active.

Vergessen has pointed out that his atk stat is so comically high that he can switch to wary fighter and play for OHKOs with glimmer. (For kicks, try baiting squishy red mages with nearly charged special spiral + astra. Don't be surprised to see over 200 damage.)

If you're not afraid of OHKOing opponents (which is a serious concern for having well over 60 atk; even squishy greens like SF Nino might die in one hit), galeforce is a good option. Pivot can be useful on him for retreating after he has taken his target, and atk smoke can help the tank who will cover him once he pivots behind.

IVs: +def, +res, and +atk are generally going to be the preferred IVs, but even +spd can be good, especially with investment. 30 spd with just the prf effect; add atk/spd solo for 36; hone armour or darting stance can get him to 42. Even if you don't invest in his spd, having 30 spd is a relatively solid bet for forcing doubles against wary fighters. (It's worth noting that L.Hector, Surtr, and possibly even Myrrh three of the units most likely to force undoubles against him, have colour advantage and probably enough defense to wall him if he's using galeforce.)

-HP: is one of the better banes
-atk: should hardly be an issue, so much atk he has already. In fact, it might be desired as a way of reining it in so that he doesn't OHKO everything.
-spd: makes hard for him to avoid doubles, which is not a huge problem given how tanky he can get
-def: not preferred, but his def is more than decent even with the bane and he comes with atk/def solo and 4 def on his weapon, so -def just means he doesn't reach Surtr levels of def without some buffing.
-res: is potentially annoying, but if he focuses on player phase it often won't matter.

Fodder options:
- atk/def solo: Akariss likes it for reasons I don't know. I find it relatively niche - good for L.Ephraim; good for Walhart...most units who could use this skill are probably fine with brazen atk/def which is more available and trades the first round of combat for freedom in positioning, which translates to freedom of playing on either phase without being awkward.
- bold fighter
- close guard
i.e. another gold mine of fodder.

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Eirika: having access to armour skills makes her really unique as a healer.

Before I go further I want to comment that I don't consider myself an expert in healers. But I do think what I have to say is worth considering.

Her low spd is definitely a surprise and affects significantly how she will want to build. People have suggested bold or vengeful fighter on her.

With atk in the mid 30s and access to atk/def or atk/res or even atk/spd solo, she can conceivably dish out some serious damage, though probably not enough against tanky opponents due to her lack of special. So bold fighter makes some sense. I don't think I find the damage consistent enough, and in any case, she's likely on a team with units who are capable of inflicting much more damage, especially armours who will often DC while having a forced double on player phase, which would make unnecessary for her to fight and much more necessary for her to replenish their HP. One argument that has been made for bold fighter is that it makes miracle charge very quickly, which is perhaps true and possibly advantageous if she uses martyr+.

Vengeful fighter is an interesting idea because she can heal on player phase and still do some damage during enemy phase. However, for having mixed bulk in the mid 30s and barely more than 40HP, I find this idea quite risky due to her low spd. Many units will double through her, especially if they can active a special. I find the miracle argument quite poor here because it will not even finish charging by the time she has been doubled. Probably the best try involving vengeful fighter is close counter + absorb+. With sufficient defensive buffs (which is not that hard when armours like stacking wards), her survivability can be quite strong.

My own advice is that wary fighter is probably best. It's not rare or expensive; it's solid, and it allows her to exploit her ability with healer staves with minimal risk of repercussion. She can use witchy wand, candlelight/flash, fear, pain, or potentially even slow with a dazzling refine. In the approach of a skill that blocks dazzle, firesweep, and the candlelight effect, the combination of wary fighter + dazzle is especially good because it's difficult for an opponent to have null dazzle AND a forced double to cancel her wary fighter AND enough spd to win the spd check. Wary fighter also synergizes amazingly with brazen def/res, which is inheritable from a GHB unit. (It's worth noting that brazen def/res does not synergize perfectly with martyr+. It may or may not, because it only heals 50% of damage the unit has suffered, so if she is at 60%HP or below, the brazen will remain active). With wary + brazen I am of the opinion that she is quite likely to charge miracle, especially if she uses fear+ or witchy wand+.

One other consideration is her healing assist.
- martyr+ can be especially good with miracle
- restore+ she comes with, and it's quite good in PvP for being able to remove debuffs and other status effects.
- physic+ deserves special consideration because she's an armour (i.e. limited movement) and comes with two buffs which are position-independent.

If you don't have fortress def/res to give her or some kind of solo skill, consider Svalinn shield. Healers don't have that many A-skills to choose from.

IVs: +def is probably best, and +res is probably second-best. +spd can be useful for avoiding doubles when wary fighter blocks a forced double such as bold fighter, and +atk is definitely also good.

-HP: probably not of much consequence, but it does mean unupgraded tactics rooms in AR will hit her unless she is receiving blessing bonuses.
-atk: impairs her damage, still leaving a huge amount of her function such as inflicting staff debuffs and healing allies. She will maybe heal 1HP-2HP less; the difference is negligible.
-spd: maybe want to be a bit careful because I know from experience with Dorcas that there is a difference between 23 and 20 spd. Units with spd in the upper 20s, which can easily include blue Micaiah, will start doubling.
-def: is very annoying. Might want to stay away from close counter in favour of def-boosting A-skills such as def/res bond.
-res: also annoying, though maybe less than -def. However, aside from ploying I don't see her attempting to change her style. She will still attempt to do more or less exactly what she would otherwise do.

Fodder options:
- Lamp post: good buffing weapon
- dazzling staff: but beware because dataminers found a skill ability that enables units to counterattack despite getting hit by dazzling staff or firesweep
- Atk opening: potentially good for AR def because the AI is bad at positioning itself to receive buffs.

NB I haven't watched Lu Bu's commentary on having an armoured healer, and he's a widely recognized expert in healers so I recommend looking up his video on YouTube.

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Comments or questions, please post in the replies!