Legendary Leif

For this low-investment Vantage build to be fully effective, Leif can theoretically take any IVs that don’t touch his HP, so long as he isn’t merged. It’s just that +Atk/-Spd is the most synergistic with this build (as well as pretty much every other build). Meisterbogen is a dual phase Brave Weapon, meaning it’s very useful for Vantage builds like this one. The assist is, of course, flexible, but Pivot can help with retreating after triggering Njorun’s Zeal if Leif is in over his head. This build is especially affordable, essentially just being Leif’s base kit with Vantage and Pivot tacked on. The intended playstyle is to attack melee enemies on the player phase, which debuffs enemies with Def Smoke, and procs Njorun’s Zeal due to Quickened Pulse, meaning you attack again (hopefully eliminating all nearby melee enemies). By this point, 10 damage has been taken due to Atk/Res Push 4, which puts Leif at exactly 75% HP for Vantage to activate, provided he’s unmerged and doesn’t have an IV with +HP or -HP. Since you’ve hopefully rid yourself of any nearby Melee units that would trouble Leif without Close Counter, the only threats left should be ranged units that Leif can deal with easily.

Preferred IV: +ATK / -SPD

Leif's excellent attack should be brought even higher with an asset, since doing so will allow him to nuke even more effectively. Additionally, none of his other stats have much relevance to his nuking capability. His speed is his least useful stat, as its 22 base (17 after Meisterbogen's -5 speed penalty) will prevent him from doubling all but the slowest of units in most situations. Because of this, his speed is the best choice to take a flaw in.

Weapon: Meisterbogen

This weapon's brave effect has the rare trait of activating on both player AND enemy phases. However, like most brave builds, this build only seeks to take advantage of its player phase usefulness. Leif's superb attack stat combines with this weapon to create a deadly combination that utterly annihilates weaker foes, and can leave heavily defensive enemies with large dents in their armor.

Assist: Pivot / Flexible

As with most units, Leif's Assist skill is very flexible. Pivot is an unexpectedly great option for him, as it allows him to retreat behind an ally with great defensive prowess if you use proper positioning. Reposition can do the same, except it moves his ally in between him and any potential threats, and leaves his ally more exposed in most situations. However, any movement Assist skill can be quite useful.

Special: Njörun's Zeal

Legendary Leif's unique special is practically a must for him in a general-use player-phase nuke role. Its Galeforce effect allows him to obliterate or heavily damage two targets in one player phase, and will trigger after almost every combat.

Passive A: Atk/Res Push 4 / Death Blow 3/4 / Heavy Blade 4

Atk/Res Push 4: This is one of Leif's best options, as not only does it increase his Atk and Res by 7, allowing him to more effectively nuke enemies, but it also is one of the skills he comes with, meaning that it's great for users on a budget. Beware of its HP draining side-effect, however, as after enough combats he will no longer be able to receive its buff if he does not get healed by an ally.

Death Blow 3/4: Death Blow 3/4 allow Leif to gain Atk during combat, just like Atk/Res Push 4, but without the HP cost that the latter incurs. Death Blow 4 gives a greater boost to Atk than Atk/Res Push 4, and does not drain Leif's HP, so it is the better option if you do not care for the Res that Leif gains from his Push skill. Death Blow 3 does not provide as much of an Atk boost as Atk/Res Push 4, however, so users on a budget will have to weight the benefits of accepting the downgrade in Atk, having two stats receive +7 buffs at the cost of HP after every combat, or sacrificing a hard-to-get unit to get Death Blow 4.

Heavy Blade 4: If not using the Heavy Blade 3 sacred seal, then Heavy Blade 4 is an alternative A slot option. Like a Heavy Blade 3 seal, having Heavy Blade 3 will allow Leif to more consistently activate his special, with the added bonus of +5 attack. However, if you don't have access to Heavy Blade 4, then it is best to stick with Heavy Blade 3 as your seal and using one of the aforementioned A slot options, as using Heavy Blade 3 in Leif's A slot will prevent him from gaining access to powerful Atk buffs.

Passive B: Seal Def 3 / Breaker skills

Seal Def 3: This skill allows Leif to take down tougher enemies that he would not be normally able to even if he attacks them twice in one turn using his special. After the first combat, the opponent will be at -7 Def, so Leif will be able to do 14 more damage in the second combat. Although this skill does nothing for Leif if he one-rounds an opponent, it can allow him to pull through in clutch situations against enemies with high physical bulk that tank his first combat.

Breaker skills: As a non-flying, colorless bow unit, Leif is able to take advantage of all Breaker skills, effectively allowing him to quad-hit any enemy as long as you change which Breaker skill he has equipped before battle. In Leif's case, these skills can only be put to their most effective use if you make use of pre-battle map viewing, or if you are repeating a previously failed map (such as a Legendary Hero Battle) and know which enemy you need to defeat with Leif using a Breaker skill. These skills are not generally what you should take Leif into battle with, but can be very useful in certain situations when you know beforehand that you will need them.

Passive C: Def Smoke

By using Def Smoke, Leif will be able to kill enemies he normally wouldn't be able to after triggering his unique special with his previous attack. By attacking a frailer enemy first, he can soften up a tougher enemy and be more likely to KO them after he gains another action from his special.

Sacred Seal: Heavy Blade / Death Blow

Heavy Blade allows Leif to more consistently activate his special every combat. If his special is at max cooldown and his opponent is unable to counterattack, then his special will not trigger. However, given his superb attack stat, he will often be able to trigger Heavy Blade and gain a guaranteed special trigger if he does so (as long as his opponent does not have special-inhibiting skills).

Death Blow: Death Blow will effectively allow Leif to deal +12 damage due to his personal weapon's brave effect.

This build is very much usable on an unmerged +spd Leif, if he's not +10 then +Atk will not do because of having not enough speed. The amount of speed you have with all your skills active and hone cavalry buffs is 47 speed, which is the target range for being fast enough without overkilling your speed. This amount of speed is the exact same on a +10 Leif without a speed boon, making +Atk more preferred if you choose to +10 him.

With this Leif can finally be the Quadding unit you've always wanted him to be.

With his superboon in speed he can reach 26 speed before any buffs are accounted for. Having Brazen Atk/Spd 3 and 4 gives him +17 speed, and if we add the lull as effective speed, in Brazen range he has a solid 46 speed.

If he attacks and then his special goes off and he attacks a unit who was affected by the smoke, he has an effective speed of 53, and with a Tactic/Hone Cav buff that's 59 effective speed. He won't ever actually need to hit 4 times though, because of the almost equal buff to his attack.

a combined galeforce+vantage build
the idea is to kill all melee enemies on player phase, and during enemy phase thanks to the recoil of atk/res push push L!leif safely into vantage range, kill all ranged enemies.
First initiation followed by a refresh and then the Njörun's Zeal allows L!Leif to attack 3 times and take 15 in recoin dmg. pushing L!Leif into vantage as long as he don't have over 60 max HP somehow.
Brazen atk/def is used to increase the mixed bulk while maximizing atk