L!Lyn andL!Ephraim build

Nothing groundbreaking, but it’s an extremely reliable build with plenty of sustainability. This is +Atk -Spd

Give Ephraim Galeforce, Death Blow (Brazen atk / def is probably best for him given his defensive bane) and Heavy Blade

Mine is +atk -res he is really useful on GHB and against legendary maps

A lot of people recommend galeforce builds on L.Ephraim. For your IV brazen atk/def as suggested by Johann is an excellent idea for two reasons: it boosts his def, and also you don't want to hit too hard before galeforce activates or you might OHKO someone. You have a good amount of atk already thanks to his atk being naturally high, his weapon granting +3, and you even having a boon. The B-skill was never obvious for him on the galeforce build. Chill def can be a good idea, or any chill to help the team, or something like def/res link to support any unit that tries to get him out of danger once he's done his thing. Less expensive options include desperation, lunge (very aggressive option), pass, and escape route.

As for L.Lyn, I like Lu Bu's suggestion of fury for extra stats on both phases. And I do suggest keeping the prf. Its condition is not difficult to activate, and I feel that she needs the extra stats. Keep desperation, and add your choice of moonbow or draconic aura. (Iceberg or even glacies might also be an idea if you buff her res at all.) The rest depends on what the team wants. With 33 res (IV of 27, +3 from prf, +3 from fury) she can even ploy some opponents.

I think I remember you liking all three reds right? You could pull reds until you get at least one focus unit.

I'm basically seeing only L!Ephraim builds, so I'll do some L!Lyn.
I personally don't have her, but I can give some advice.
First off, +atk -hp is insanely good lol. Although 48/36 is underwhelming, with team support and her weapon she can easily get up to 57/45 above. With building, I'd keep Despo and Spd Tactic. Moonbow or Luna is a good option for her, and her base dual rally is a great choice, or Reposition if you desire. For SS, probably Res or Def Tactic 3.
I've saved the hardest for last. Yes, Laws of Sacae is a nice sounding skill, but her bulk is so low it's futile trying to make her into an enemy phase unit. You can if you want and change Despo to QR, Guard, or Renewal, but since you have IVs encouraging a PP unit, go with Fury; or, if you're up to it, L&D, though I highly do not recommend the latter. Swift Sparrow is a good option if you want to save HP and have the units to supply it.
All in all, great unit, just needs a lot of team support. However, Lyn's not the type to take and not give, as she dishes out an incredible +6 Spd/Res or Def buff to allies within two spaces that have 2 or less of the same movement type on the team.
All in all,
Lyn: Lady of the Wind
33 HP/48 Atk (equipped)/36 Spd/21 Def/29 Res
Weapon: Swift Mulagir
Support: Rally Def/Res or Reposition
Special: Moonbow/Luna
A: Fury 3/Swift Sparrow 2 (L&D?)
B: Desperation 3
C: Spd Tactic 3
SS: Res Tactic 3/Def Tactic 3