"I pulled Ophelia/Flora/Nina/Silas +this -that. What do I do?"

Table of Contents
1. Ophelia
2. Flora
3. Nina
4. Silas

Ophelia: best as a one-shot mage. Ishtar, Linde, Lute, and Tailtiu all have her outsped, and her weapon lends herself to it. She even has a superboon taking her to 41 atk before weapon or skills. If she has no mage allies or infantry pulse support, give her glimmer. If she has mage allies, draconic aura or AoE specials are good. If you're scared of vantage, use hardy bearing. Level 1 is probably enough because she'll start the battle at 100% HP. If you're not scared of vantage, L&D is probably ideal unless you're not using an AoE special, in which case death blow 4 is probably ideal.

Ideal is probably growing or blazing wind + special spiral, but if you don't have special spiral, her spd is actually decent, especially with a boon. But you can opt to stick to short battles (i.e. no grand conquests or PvE battles involving too many reinforcements)

IVs: +atk best, +spd second-best. bane in any defensive stat including HP. -spd is more or less inconsequential on OHKO builds; -atk is annoying on all builds but she'll probably blow through everything on a blazing AoE special anyways, especially with chill res to help.

Hyper-budget options: base kit, QP seal with one mage ally to have the AoE special right at the start, any C-skill to support the team (preferably not the team's only hone atk), maybe switch to atk+3 in her A-slot if you're keeping the AoE special because sturdy blow won't count out of combat. If you're supporting her with infantry pulse and a mage ally, she won't need QP so you can use hardy bearing 1.

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Flora: use her base kit, or swap out QR for guard if you don't care about baiting mages, but contrary to Lu Bu's suggestion of guard I like her ability to provide coverage. I do like his suggestion to use flashing blade so that she can hit DC units with glacies, but based on her atk stat and native A-skill and def ploy and my experience with Felicia, iceberg is probably enough unless she's against a blue. I recommend spd ploy for a seal, both to help herself and her teammates.

Spd is a sensitive stat for Flora. -spd is entirely workable but definitely not ideal. I actually like +spd because you automatically block doubles from most Lilinas, Sanakis, and Micaiahs, especially if she's outmerged. And she might need to do it with base spd, because she can't necessarily get a spd ploy off on Micaiah. Also, +spd combined with spd ploy gives her a good chance of cracking wary fighters using the forced double on her weapon. If someone else is using the spd ploy seal and she doesn't have +spd, then I think +spd seal is a good idea.

IVs: +atk is very good; +res is about as good if you want to bait mages, +spd is good. -def and -HP are preferred, -HP more than -def because with 38 def initiating against DC, she might actually survive two counterattacks in some cases. 34 def might still pull it off, but less often.
-atk is probably not a problem against squishy mages, especially with the iceberg, but be careful against DC units because they tend to have high def. You still have 51 atk with atk/res solo and then def ploy and a 19 or 22-damage iceberg on top of it, so if they have 50HP and 40 def before the ploy you're still good, just barely.
-res is actually a little sketchy for baiting mages if you don't use atk ploy, and extremely sketchy if you're going to get hit with a moonbow. Well, it was always sketchy if you get hit with a moonbow, as the quickened moonbow Reinhardt vs Felicia matchup has demonstrated long ago. (If you're not familiar with it, Felicia will usually lose if she's not on a defense tile despite having 35 res without an A-skill. Guard is necessary to bait Reinhardt reliably.) 35 res is probably still enough to land the def ploy. A red mage needs to hit twice with 60 atk if there's no moonbow, which a blade tome or potentially even Ragnarok+eff Celica or an AoE special Lilina can do so be careful. Res bane isn't ideal but is probably fine if you proceed with caution.
-spd I already discussed. Beware of slow mages and wary fighters.

Hyper-budget options: I would expect the base kit to work pretty well with any IVs, actually. Seal options were discussed above.

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Nina: is not good with her default bow because her statline shows a clear preference for fighting mages, who will almost always have higher res than def, making shining bow a worse silver bow.

If she wants to play aggressively with her 32/37 statline (which is not a bad option), slaying bow is probably best unless you have +atk, in which case brave bow or firesweep can be considered because it's almost like having +spd B.Cordelia/S.Takumi. The nice part about Nina's statline if you have +atk without -spd is that she can use fury and still ploy everyone that isn't a candidate for using ploys themselves, so you can do something like brave bow, fury, spd ploy and have 40 effective spd. (Finish the build with desperation, luna, some assist, and spd+3 or def ploy.)

The other option is to play her as an enemy phase anti-mage with QR to deal with stuff like Celica or Nino+10, in which case guard bow or gratia are the weapons of choice. Get her spd up to 42, or if you really want to compete, 45-46, and use QR. Distant def, atk ploy, spd ploy, spd+3, and even res+3 are all seals to be considered. Like Flora, she can get her res up to around 50 with guard bow and, for example, distant def + atk ploy.

IVs: +atk is probably the preference for offensive builds. +spd is good for all builds. +res can be helpful for ploying, but I would say it's unnecessary because 34 res is probably enough to ploy the targets she wants to ploy (super-fast mages don't reach 34 res even at +10) so a res refine on her weapon will be enough. If you have +res you can probably get her res to something like 50 though if you count an atk ploy. def and HP are the desired banes.
-atk is not good for aggressive builds. You can buff it back up, or you can stick to baiting mages. Felicia was busting squishy mages with 42-ish attack even before she was getting extra charges from the Plate, and Nina has the spd to compete with that kind of super-fast mage and not get doubled.
-spd is not good for aggressive builds unless you have +atk, in which case brave bow can be considered. This bane is probably the nastiest of them all, actually, because she really doesn't want to get doubled by some glass cannon mage who probably has a lot of atk along with high spd. But with spd ploy and fury she's already at 42 effective, which will block doubles from most Nino+10, so -spd is definitely workable. I didn't even mention the weapon refine.
-res means you'll probably need fury if you want to ploy. It is relatively inconsequential on offensive builds (ex: +atk -res just do some brave bow or firesweep thing, you just won't be able to ploy), but it is unpleasant on enemy phase builds. However, unless you were planning to get her res almost to 50, her QR was probably busted after the first fight anyways, and -res probably doesn't change that, so you can proceed as normal.

Hyper-budget options: keep her weapon because her atk is a bit low and you don't want to work with a 4* weapon. If you don't have a spd bane, spd ploy + spd/res link is actually very viable to get her up to 48 spd before A-skill. Super-cheap A-skills include fury 2, darting stance 2, and spd+3. Use either a spd or a res refine on her weapon, keeping in mind that some mages get to 46 or higher spd.

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Silas: actually has a lot of options. They can be categorized for the most part using two criteria: def-focused vs mixed bulk, and spd-dumping vs 30-spd.

Def-focused builds tend to keep the base kit. You can get over 50 def with def wave or fortify cavalry, and if you use guard you just might maintain your guard forever.
Mixed bulk builds tend to go for Berkut's Lance and close def.
Spd-dumping builds generally want -spd IV and guard.
30-ish spd builds are probably good on +spd IV with the help of a weapon refine or A-skill and make the most sense either by intending to initiate against slow armours because he comes with slaying spear, or by taking mixed bulk, because a lot of manaketes sit right in the 27-34 zone and won't be able to double him.

All boons are good on him; HP is the least helpful. He's surprisingly flexible for having a statline focused on atk and def.
+atk is helpful mainly for player phase builds designed to pick on slow armours by getting to 33-ish spd
+spd is for those 30-ish spd builds I offered.
+def and +res should require no explanation.
-atk is not crippling. If you find he can't get enough damage, take a reprisal lance or use something like slaying ignis.
-spd is good for the spd-dumping guard builds
-def is annoying but by no means unworkable. Might be more favourable for a player phase anti-slowtank build or a mixed bulk build, but with base kit he still has 45 def before weapon refine when def wave activates.
-res is fine for a def-focused build.
-HP is relatively inconsequential. He probably expects to get hit multiple times, and he'll still have over 40HP with a weapon refine.

Hyper-budget options: base kit, QR2, bonfire or ignis, close def or fortress D. This is the "I have so much def I'll straight up tank your special" build. If he has hone cavalry support or +spd IV, it's also an option to refine his weapon for spd and use a spd seal (even spd+1 will sometimes be enough) to take bold fighters on player phase and everyone else on enemy phase. If you're doing that then switch to moonbow in case B.Hector's def is just too freaking high.