Leo

A -SPD IV is strongly recommended against, as it strips him from a lot of his bulk by making him doubled by practically every unit in the game. +ATK is the only viable +IV for this set, since it hinges upon high attack power and doesn’t rely upon his defenses as much.

The biggest mistake most people make when using Leo is assuming he’s supposed to be built around OHKOing units. Actually, Leo is a highly effective sword killer. Even at -DEF and carrying Life and Death, with Swordbreaker Leo survives and kills every single neutral sword unit (yes, this includes the Distant Counter sword units), with the exception of Eliwood, Fir, Sigurd, and TW!Reinhardt. With a simple buff, Rauðrblade lets him ORKO every (not Sigurd) sword unit in the game.

With Rauðrblade, Leo is also able to one-shot a decent chunk of the cast with the correct buffs. It should go without saying that he excels with cavalry buffs, but he’s quite lethal in non-cavalry teams too. His cavalry mobility provides him a distinct advantage over other red mages looking to do the same job and synergizes well with Draw Back.

Death Blow can be picked over Life and Death; Death Blow preserves more of his bulk, whereas Life and Death allows him to double slow units he otherwise wouldn’t have been able to. It’s up to personal preference and playstyle.

The C slot and Special also come down entirely to personal preference. If you’re running Death Blow over Life and Death, Res Ploy is a good choice for the C skill. Otherwise, Atk Smoke or a Drive skill is probably the best choice. Savage Blow or a Threaten skill also work well for those who don’t have access to Res Ploy, Atk Smoke, or a Drive skill.

For Specials, Draconic Aura will deal the most consistent damage, but Leo can also run an AOE skill pretty effectively. With Swordbreaker, his AOE will charge fairly quickly and Rauðrblade+’s +1 cooldown charge time doesn’t matter with an AOE skill.

For maximum effectiveness, Leo likes having a Sing/Dance unit on his team, letting him kill multiple sword units in one turn or finish off a unit he couldn’t OHKO. And while he is able to survive and kill Ike, Xander, and Ryouma, they knock him into low health, which strips him of his Swordbreaker ability, so he likes blue melee teammates to deal with them instead. While Leo can kill quite a few non-Distant Counter, Triangle Adept/Sapphire Lance, or high RES lance units in two rounds, blue mages give him trouble and he wants a teammate who can handle them, too.

While Leo can’t kill every unit in the game or win every matchup, no unit should be expected to. But he also doesn’t require a team to be built around him and is a welcome addition to almost any thought-out and balanced team, providing magical offense, high mobility, and a reliable sword counter.

This build is for those with a plus defense IV Leo and who don't have or want to use Brynhildr's refined effect.

This is a rather expensive build given how Leo is to change his weapon, but in return he will excel in battling colorless and green units with ease given his health is at or above 70%. Obviously keep him away from blue physical and magical units and horse killing weapons.

This build requires Leo to be buffed in every stat in some form or fashion because Leo's statline and native Quick Riposte 3 restrict him to be an enemy phase unit.

A Leo build geared toward scoring as much as possible and facing the armor meta.

In the higher tiers, ranged units are almost non-existent so Brynhildr's refined effect is useless. Since Leo's unique weapon is still taken for maximum points and Leo's Attack is not in OHKO range, this Leo is reliant on Quick Riposte-Close Counter-Close Def baiting, hence the choice of a +Def refine for more bulk.

Drive Spd 2 is a 240 SP skill that comes with Rally Spd/Res from a four star Tailtiu. It is not particularly useful in my team but it does its job of putting Leo's SP total over the next hundred for an extra unit arena point.

Seal: Distant Def
Assist alt: flexible
A alt: Fierce Stance, Atk/Def Bond, Brazen Atk/Def, Fury
C alt: flexible
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Disclaimer: I've know Leon as Leon before he was localised to Leo so proceed with caution.

In light of the new weapon forge, Leon's tome has given him an...underwhelming effect compared to the others. However, this strongly specialises him to be very good at his one job.
Sturdy Stance is taken to help improve upon his atk and def as these are largely the stats that will matter in his new role as a mage baiter. His 30 flat res with Brynhildr's 30% reduction on magic turns his magical bulk into something very respectable. Other A skills could be Fierce Stance for more damage output, Atk/Def Bond to allow him to have some offensive presence during Player Phase as well. Brazen Atk/Def can offer more damage output when he's 31HP downwards and Fury is the ever-reasonable budget alternative but both have the caveat of requiring HP managment since Quick Riposte is nearly mandatory.
QR3 will break at roughly 27/39 HP which means setup with Ardent Sacrifice isn't possible for Brazen, and Fury's tick damage only allows for 2 damage to be sustained before breaking. However, if endurance isn't the game you're playing, him eradicating a single ranged unit very well and serving as a decoy tank for other ranged units can make these iterations workable. Other B skills could possibly include...well, none really, barring a breaker if you really want him to endure dealing with a single particular ranged type for longer. But he comes with QR3 so why squander it?
C skill is largely flexible but Atk Smoke was assumed due to it activating when he's attacked and can help with setting the next player phase for his allies. He can also use this to attack (somewhat) safely without worrying (as much) about breaking his QR threshold if he needs to throw himself at another foe.
Distant Def as his seal should be self-explanatory: it adds to his bulk of the relevant range type he wants to be provoking.
Moonbow is chosen because of rhythm lets him to have it proc on his second hit and it cuts the opponent's res which is crucial as he's packing the ferocity of a dandelion puff.
Support skill is also flexible. A dual Rally is chosen here for both arena weight and allowing himself to still be helpful to his allies. The standard fare of Reposition, Draw Back, etc... are also equally viable here.

Mounted Mage Terror
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Leo serves as the red cavalier mage in a bladetome horse team (there’s no one else as of writting this).

With close counter and vantage, leo can dish out excellent damage when attacking or attacked.

The newly released quick reposte sacred seal complements this build as well, it’s a matter if patching up leo’s decent defenses or guaranteeing a kill when attacked above 70%.

Asumming +10 merge, this leo can reach 39 def/40 res from a fortify cavalry, or 45 def/46 res if using close/distant defense, which is pretty decent for a mounted mage.

As long as the enemy doesn’t pack the hardy bearing seal, or the firesweep weapons or watersweep or is a forged Jaffar, most of the units will have a hard time killing Leo if he survives below the hp activation threshold for vantage.

Leo acts as a tank alongside Camus in my Bladetome Cavalry team, all that I’m lacking is close counter.