Leon

Ooh, baby. You could be a sweet summer child, using a basic skill like Ignis, or Luna, or what have you.
Or! You could be full of life and joy, and pair up your favorite fashion model and hand him a sicknasty duo of Astra and Heavy Blade.

With 50 attack, there’s a high chance you’ll pull off your heavy-blading. With a typical Hone buff, you’ll almost definitely get your charges. Pair with Lancina, or perhaps Forehead Princess™, and you’ll reach even greater heights of offensively offensive firepower.

Observe as his hail of death on the battlefield glimmers with astral lethality, able to quickly shred just about anything as long as it’s under 35 or so defense, and even then you’ll still be packing some serious heat to punch through a hefty chunk of their health.

As for B-slots:
Cancel Affinity prevents even the hardiest of Adepts from dissing your fly girl.
Alternatively, keep your guard up and you remain immune to accidental backlashes of Black Luna, Aether, and other mistakes while attacking someone with Distant Counter, which Leon can thankfully shrug off one or two hits without collapsing like his peers.

And as you’re likely aware, our runway model already has so much magic in is heart that even more will cause an overdose and prompt him to faint, so remember to keep your Leon safe while engaging dangerous folks like Tiny Hands McBastard and his usual friends.

Go forth and dazzle the enemy with your brilliance!

Sacred Seal: Deflect Missile 3

The one goal of this build is to allow Leon to check archers. Guard Bow+ refined for Defense will provide Leon with +3 def and an additional +6 def/res when attacked by bows or daggers. Attack boon will Leon maximize damage dealt. I recommend Fury 3, Defense 3, or Distance Defend 3 as Passive A. Bowbreaker 3 to prevent enemies from doubling and allowing Leon to double. Passive C is to be your preferred skill. Deflect Missile 3 as sacred seal allows Leon to survive Brave Bow+ archers equipped with Moonbow plus Quicken Pulse.

Seal:
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GODDAMN YOU JOKOB WHY DID YOU HAVE TO EXIST I WANTED HENRY BUT YOU KEEP EATING ALL MY GREEN CHANCES LIKE THE FUN RUINER PIECE OF SHIT THAT YOU ARE

Having a much better attack stat than Gordin, he can run this build better than him. Unfortunately though trying for a quad striking build with Leon is ill advised since it's not high enough to take advantage of it, meaning this is his only strong build with the Brave Bow+

Semi-typical Firesweep build. B slot can be switched with Cancel Affinity if you're not afraid of Nino and other fast green mages. The C slot can be almost anything that is not a threaten skill as long as it works with what your team needs. If you plan to use summoner supports on him, you can use Attack boon, otherwise Speed boon is necessary. Bane is preferred to be Resistance, but any of the defensive stats can take a hit since Leon is not meant for being in the line of fire on enemy phases while using firesweep.
With this build, he is best paired with a dancer that has hone speed to assure he can double and kill faster units. The dancer is there mainly to give Leon options of killing a 2nd enemy or retreating.