Robin (F)

Aight so Im just going to say upfront that this is a incredibly expensive build, however the rewards for this build are quite nice since it makes Robin *viable* making her a high damaging tank (sorta).
A Slot: Close Counter to make use of Robins defense stat against melee units.
B Slot: Def/Res Link was originally meant for support but since it allows infantry to be able to get +6 to 2 stats without a tactic team it was a nice coincidence and it's necessary for a blade tome.
C Slot: Spd Wave (Atk is also fine but you gotta fill the gap in her Spd) its just a nice added bonus for every turn since you wont need an ally to get it off. Seal: Darting Stance is added to close the gap in her Spd but that is my prefrence, especially since you could and probably should use quick riposte if youre not using SS but you have to keep her health up.
Special: Aether isnt necessary or even the best choice but if you have infantry rush it becomes quite viable for its healing and damage imo. With support from other units you can get all stats to +6 making her more threatening then Nino because Robin can take a hit, at the end of the day you would only do this if you like Robin way more then meta characters with nice skills (Karla, Nino, Winter Tharja and others lost their lives to this build) but Robin can one shot people so its probably worth it

Tactical gale combined with mystic boost will allow her to survive attacks from staff users, with mystic boost also giving a small amount of healing in combat distant defence also help with survivability especially with attack smoke.
This build is primarily used as a buffer for your team, trying to stay alive and out of combat as much as possible to give buffs to other teammates for as long as possible during a match.

If you reaally like Robin you can try this set up. An ultra speedy girl is here, having 50/41 offencive spread with summoner support to have 47/50/41/26/19 hp/atk/spd/def/res at 0 merge.
With that stat line she is basically red M!Morgan without legendary tome but thanks to "silver" tome she enjoys as much stat gain as she can get. In order to increase her builk i chose atk smoke as it gives her +7 to def/ res increasing those stats of her from 26/19 to 33/26 wich is great in combination with 47 hp (42 if not summoner supported).

Robin is a skilled tactician with deadly powers that she is unaware of.
This build makes the Fell Grandmaster very powerful while staying true to her source in her original game from Fire Emblem Awakening.

Blarraven helps deal with any colorless unit like bow or dagger units.
Harsh Command is a substitute to Rally Spectrum where she’ll convert her allies’ debuffs into stat boosts.
Ignis is a special skill that she always gets in her original game.
Death Blow helps her hit hard like a truck.
Poison Strike and Savage Blow helps deal extra damage and references her Fell Dragon Powers (even though she can’t get Savage Blow in her original game)
And Resistance Tactic is good to have on a tactician like her.

Make sure she is able to control her powers and doesn’t turn into the Fell Dragon, Grima.

Robin: Mystery Tactician
Debuff Support Mage

This Robin build is meant to be used on an Arena Defense team as an annoyier unit. Through the utilization of double Panic and Smoke, Robin can potentially cut a teams offensive abilities by more than 10. Though the build can be better run on a number of other units with better merge potential as well as optimizible stat lines, Robin is a F2P mage that also has good DEF over RES that few other mages have, as well as a beloved character by many.

Using Spectral Tome makes for some interesting tactics which may catch some players by surprise with the Panic status. The tome allows for Panic to be spread regardless of the HP stat as well as beyond the cardinal directions. A RES refine is used to balance Robin's defenses in order to cover a wider range of opponents, though a DEF refine alongside Bonfire/Ignis can also be selected. It is also possible to pair Robin with F!Grima and use a SPD refine to potentially make her horrible SPD usable during the Earth season.

Reposition is used because it is. At this point in the game you should know by now that Reposition is the most versatile assist skill out there.

Moonbow is used for its low cooldown as well as ability to cut through some opposing defenses. It can also be replaced with Luna/Bonfire/Ignis for more damage. Robin doesn't hit very hard, so keep this in mind when selecting a special.

Fury is used for the A-slot, as Robin appreciates the additional +3 to all of her stars. Her SPD needs it the least, but as aforementioned, alongside a SPD refine and F!Grima, it can be made manageable. Another potential skill is Close Counter, which Robin can use fairly well due to her good DEF stat; this also allows her to spread Panic and dual Smoke regardless of the attacker, so long as she isn't OHKO'd. If Fury fodder is absent, Earth Boost or a flat Stat Boost skill can also be used.

For her B-slot, Guard is used to further annoy opponents, especially Blue, Green, and Colorless units that may look to sure-up a KO using their special. Green Tome Breaker can be run on a budget as well as to target specifically high merge budget Bladetome users like Nino and Cecilia. Her stock Blue Tome Breaker can also be used, but outside of Reinhardt, few blue mages exist in the meta, and you're more likely to be KO'd by Reinhardt when running BTB than with Guard. If not running Guard, you're better off with GTB.

For C-slot/Sacred Seal, dual Smoke skills are run, with ATK and SPD Smoke being the best choices. The best part about debuffs and panic is that they ultimately stack, and can lead to some sever circumstances for the opponent. With ATK and SPD buffs being the most common in Arean Offense teams, doubling down on them can put the opponent in a tight spot. In it's most extreme instance, an opponent with +6 ATK/SPD, they will come out of the bout with -6 instead, and other units will also get the -7 effect, issuing a grand total of -13 to both stats. It won't matter if Robin survives the battle should other members of your team be able to finish them off. The other two Smoke skills can be run in place to allow for better offense on your teams player player phase, but SPD smoke benefits both phases and should almost always be run if possible.