|Leif is an incredibly flexible unit overall, his 32 base Atk is workable with his weapon, and can be patched up with either a
+Atk IV or an Attack +3 seal, his 38 base Spd is topped only by Mia: Lady of Blades and Lon'qu: Solitary Blade, 40
HP is neither so cumbersome that he'll never reach desperation range, neither so tiny that he's danger of being 1 shot super easily, 30 Def
makes bonfire workable on him with Fury and/or a +Def IV, and whilst 23 RES kinda sucks, plenty of top tier units don't care
( Ayra: Astra's Wielder says hi) as long as they have enough defenses or other ways to survive then kill at least certain types of
Weapon choice here is obvious, Leif really wants the Light Brand to patch up his rather "meh" Atk stat, and he doesn't really have the Spd to make a quad build work without a +Spd IV, thusly, we stick with his base weapon, as this build is meant to show off the many things that Leif can do. Whilst I could see an argument for a Brave Sword+ build, this build is already effectively 2 in one, so I have no intentions to bloat this further.
His assist skill is also obvious, as a melee unit without some particular niche, Leif should always take Reposition.
His special is where things start to get muddied. Depending on any given Leif's IVs, if you have the will to merge him up, and what A skill you will be running, you'll choose one of 4 different specials. Bonfire is your standard one, if you have a +Def Leif, this is probably what you want, if you intend on running Fury 3, this is almost certainly what you want, if you are going to merge Leif up, this is probably what you want. Draconic Aura is a solid choice, if you manage to have the Light Brand's effect go off, even with neutral IVs and no Fury, it should add a whopping 17 damage to your attack. If you got -Def on your IVs, have no intention on running either Fury 3, Warding Breath, or Steady Breath, or just don't have the fodder for Bonfire or the upcoming choice, Draconic Aura will help you out. Glimmer is, IMO, the best choice for Leif without +Def IVs and Fury 3, and is definitely the best choice if you intend on running either of the Breath skills, as it could help you pierce through some of the HP tanks and will charge in just 1 hit if you are running a Breath skill. Moonbow is never a particularly bad choice, but Glimmer tends to do its job but better. Run it only if you don't have fodder for any of the other skills. Just as a side note, Ignis would also work if you are running a Breath build, but at this point this entire thing is way too long, and I have to stop typing eventually.
Alright, that was a wall of text, hopefully his A skill is a little bit easier to parse through.
The obvious and all-around choice for Leif's A skill is Fury 3, it adds stats for, well, everything, and if you want Leif to only really be a CC/TT unit, then it works really well with S Drink. However, given his solid Def stat and far from terrible everything else, either of the Breath skills will help Leif survive those pesky mages and retaliate in full force. If you have a -HP Leif, running Warding Breath will bring Leif's Res up to nice 27, enough to tank plenty of mages in combination with Deflect Magic 3 as a seal. Even with -Res, Warding Breath and Deflect Magic 3 are still valid choices to help Leif's Enemy Phase viability. If you run a breath skill, I'd recommend either Glimmer or Ignis as your Special proc, depending on your fodder choices and/or Breath skill/IVs. If you run Fury, Bonfire is definitely what I'd recommend. But this also excludes Death Blow 3, another option if you want a player phase focused build.
Leif's B skill is also a bit difficult to parse through, with a +Spd IV and/or Fury 3, Desperation 3 allows Leif to take out certain really tanky units that would kill him on retaliation, Hector: General of Ostia is a good example if you have an unfavorably low HP total, same with Amelia: Rose of the War. S Drink is an obvious choice for CC and TT. Wrath 3 could be used to help buff his special procs. Windsweep 3 is always a possibility for defense teams if you want to catch someone off guard. Breakers help his match ups against certain trouble units. But overall, the B slot is really flexible for most builds, but Desperation 3 is always good with Fury 3 or Death Blow 3.
Leif's C skill, like with a bunch of characters, is super flexible. His inbuilt Drive Atk 2 will be a good option for those without other options, but any hones, tactics, threatens, ploys, spurs, drives, etc. are also options.
As far as seals go, Leif could do with a lot of them, Attack +3 patches up his Atk stat, Deflect Magic 3 helps his survivability against mages (like a certain caped not crusader with a brave blue tome), Quickened Pulse is a little redundant when S Drink is an option, but it's not terrible, and they, afaik, stack, Distant Def 3 would help his survivability against mages that just want to hit him once, but very hard, and Heavy Blade 3 helps with Death Blow 3 builds, maybe even making him into a Galeforce user.
Overall, mixing and matching a bunch of these to find the one that best fits your Leif and his IVs is probably the best option, I personally have a +Spd/-Res Leif, and will be running Reposition, Glimmer, Death Blow 3, Desperation 3, Drive Atk 2, and Heavy Blade 3. So I guess that last part there is my actual recommended build, huh.