"I pulled Loki/Kliff/Owain +this -that. What do I do?"

I try to cover as many versions of this question as possible, so it does get long. Take what applies to you and don't run your head straight into information overload.

Anyone with improvements to suggest, please go ahead. I do edit stuff in.

Please note that I've done this one in shorter time than some of the other ones so it might not be quite as good. I'll be polishing it over the next few days.

Kliff:
Interesting unit. His weapon suggests that he probably wants to combat units that don't have that high of res (even with 30 res he starts to have trouble getting through), which often means they're the type that's all atk and def or they're fast units who will push their spd well into the 40s. For the second category, since he's probably going to be outmerged (and/or against inflated stats) and his opponents are using A-skills that boost spd, he might want to switch chill spd for QR, especially if he he doesn't have +spd. For both categories, res ploy is advisable - against low-res units, because they might have a lot of HP, so even 20-ish res might save them. Spd ploy is also a good idea if you switch out chill spd. If you switch to QR, the distant def seal would make him extremely tanky.

If you want to tackle bulkier targets, I would say that flashing blade bonfire/iceberg stacked on iceberg is a good idea. If you have the fodder, special spiral glacies is his most ambitious try, and he maintains the option of using QR as a seal.

In all cases I like his native A-skill and don't see much point in switching it out. I'm not convinced about swift sparrow because his ploys won't work nearly as well, which means he ends up being better off with fortress def/res and ploys.

IVs: some aspects of the statline are relatively sensitive, mainly atk. Pay attention to the HP and res of opponents you expect him to win against, and make sure you have the atk and spd to do it as well as the res if you want to ploy them.
+spd is probably best followed by +res and +def. +atk is arguably better than +def and +res. -HP is probably preferred.
+HP: doesn't change anything.
+atk: most units will have 50 atk, especially if they initiate on him, especially when inflated stats are in play. The extra damage will help. Proceed as normal.
+res: nice for ploys
+def: good because he probably doesn't use desperation.
-atk: irrelevant on a special spiral build. don't use fortress seals, and have an atk buff ready. You might get in trouble if you can't find atk in the upper 40s after his weapon effect activates.
-spd: difficult IV. Even with spd ploy he might get doubled by fast units. One option is to use a spd seal. If you can't buff his spd or get him some ally support at least (spd ploy + weapon effect + ally support gets him to 40+ spd, which may be enough) then you might want to switch his A-skill to something like atk/spd bond or kestrel stance, in which case the atk from the A-skill will compensate for the damage he doesn't get from res ploy.
-def: doesn't change too much. One of the better banes.
-res: use fortress res seal if you want to ploy.

hyper-budget options: without -res, base kit, spd ploy, res ploy, moonbow. With -res, depends on the boon. +spd can get away without spd ploy so flashing blade iceberg or possibly even just atk+3 seal can get it done. +atk will lose consistency on the weapon effect if you also use atk+3 seal, so spd+3 might be an idea. And then there's fortress res seal and spd ploy.

Fodder options: he can work with more or less any IV if built and buffed accordingly, but he does have great fodder.
Fortress def/res is amazing on most units that previously liked close or distant def (especially ranged units or fliers who couldn't use breath skills).
Chill spd is good on support units and on firesweepers, especially lances who want to win against hyperspeed swords.

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Owain:
Basically slaying edge Ayra but not quite as fast. So mostly anything that's true of Ayra is true of him except that the spd is overwhelming.

Enemy phase options:
base kit, can switch atk/def bond for DC or some brazen.
Also interesting is special spiral + aether. He's pretty fast which makes him hard to double, which means this build makes him very hard to KO without bold fighter or swordbreaker.
Both builds probably want atk smoke.

Player phase option: will require strong tactics because his special is basically a bond skill.
- Missiletainn, blue flame, atk/spd bond (or any bond involving spd), special spiral. Since he gains two charges on an enemy's counterattack, this is basically Ayra's double wrath-RA except it works with dance if you can keep the bond effect active. (Otherwise it's a significantly worse version of Ayra.) A more independent, less ambitious version would use swift sparrow and bonfire instead. (Again, it's a worse version of Ayra, but a galeforcer would need over 65 atk to equal his damage, so it can work.)
- Missiletainn, swift sparrow, wrath (so requires one infantry pulse for support), aether. He can also try to dual phase this build using flashing blade, which is kind of a faux special fighter and probably wants +spd for an IV.
- Desperation builds are possible but don't exploit his prf much. Ayra is worse with desperation than Karla, and Owain can expect to be still worse.

Any build which expects him to fight multiple opponents per turn (even if on different phases) should probably use atk smoke. Other options for C-skill include spd wave and, for player phase builds, breath of life.

All builds that don't require other seals do well with spd+3, like Ayra.

Alternative interesting ideas for A-skills include brazens other than atk/spd. (Brazen atk/res seal can stack on it.)

IVs: stats from most to least important are spd, atk, def, res, HP.
+HP: changes almost nothing
+res: could be useful for DC
-spd: might want to focus on enemy phase OHKOs.
-def: can help him activate wrath against greens or low atk reds. You might switch out the def bond.
-atk: annoying, but nothing really changes except he hits slightly less hard. 25 damage on blue flame still requires a red opponent to have 70 physical bulk to survive one hit if wrath doesn't activate and Owain doesn't have any atk from his A-skill or the ally behind him. Once the A-skill and ally support are factored in, a lot of sword infantry and will still fall in one hit, as well as most ranged units if you use DC.

Hyper-budget options: base kit, spd+3 or close def seal. Works on all IVs.

Fodder options: this guy will work with any IVs, but the fodder is good too.
- Blue flame looks promising for units like Legault who lack a stat to use for a burst special. The problem is that they tend to be player phase desperationists because the criterion I offered means their atk, def, and res are all mediocre. I'll write more about this special later, but I find it tends to be good on enemy phase units because it's basically a bond skill (and a killer one at that) in the special slot.
- Wrath is good on units who can take a hit, because you won't be able to use desperation.

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Loki:
I'm not an expert on healers, and she's a bit of an odd one.

If you don't have dazzling staff, live to serve (level 2 can get the job done a lot of the time) might be your best option.

Choice of other skills and seals is highly dependent on role and IVs. Some things to point out:
- her res is not especially good for ploying, and it might not matter when the unit she's hitting with the gravity ploy might not be able to reach your army anyways. But if you want to ploy, you probably need +res and either fortress def/res or fortress res and a little merging. 34 res will hit most offensively oriented ranged units. If you ploy, atk and spd ploys make the most sense to me.
- you can double up on ploys with panic ploy, in which case keeping C duel and using an HP seal would be good ideas. Now even armours are liable to get panicked. But again, if they can't even reach you, what's the point? So it would be for restricting their movement to help bait. If there are multiple ranged units on the enemy team, the strategy is iffy. You might prefer to panic one and gravity the other so that the panicked unit attacks into you while his teammate gets stuck.
- nothing wrong with spurs, hones, or drives

IVs: matter more on her than on most healers unless you can give her dazzling staff or decide to lay aside the prf.
+spd is probably preferred if keeping C duel or using an HP seal. If you use neither, you can quite easily miss the gravity ploy on highly merged horses, especially if seasonal blessings are involved so...
+HP is a good idea if you're using neither C duel nor HP seal. The requirement for the gravity ploy is slightly more stringent than that of panic ploy 3.
+atk is kind of tasty. Her damage isn't especially intimidating, but it's not to be laughed at, and 1-2 extra HP on healing is always nice. The best boon for any vantage setup.
+res: can open the door to ploys.
+def: probably doesn't change much unless it's covering an HP bane. 58 neutral physical bulk is pretty good. Unless you're trying to tank a hit from a 60atk armour, not necessary. In any case, proceed as you otherwise would.

-res: preferred unless you want to ploy. Keeps her physical bulk and her HP.
-def: most of her bulk was in her HP anyways. If you're concerned, keep C duel or use an HP seal.
-atk: wasn't easy to kill stuff with 47 atk and no special, and you're only losing 1-2 points on healing. Not the worst thing.
-HP: can cause issues for the gravity ploy. Might want to keep C duel and use an HP seal.
-spd: can be sketchy. She'll survive one hit without much difficulty because she has so much HP, but it's important she doesn't get doubled. Consider a spd seal. Atk/spd push might also be good because she probably survives only one hit anyways. If you're scared, maybe give her physic+.

Hyper-budget options: HP seal and maybe spur def/res. Or panic ploy seal and keep C duel.

Fodder options:
- Odd atk wave: lots of different opinions on this skill, so look around. I personally recommend it for two situations: one, when trying to strike fast and take camps on turn one in GC (can be especially good on a unit potentially placed next to a galeforcer because you can keep the unit away if your galeforcer would OHKO the opponent and fail to galeforce), and two, on units that don't need it against many of their opponents but enjoy the boost to help them against some of the marginal matchups or occasionally to overcome unfavourable matchups.
- Earthwater balm is deluxe. Use it if you have some healer you love and want to spoil.
- At the moment, C duel isn't worth foddering unless you have a personal interest in arena ranking or scoring. The rewards really aren't worth it - I explain the math here :
https://fireemblem.gamepress.gg/q-a/cold-hard-economics-duel-skills