Leo

+Def/-HP

A boon in attack is just as good as a boon in defense to help alleviate Leo's mediocre attack. A defense boon, in addition to being a super boon, allows a more comfortable range for Brynhildr's (+Eff) effect and adds to Leo's overall tankiness. Although a bane in speed is preferred, -HP isn't bad.

Brynhildr (+Eff) is better than vanilla Brynhildr for an enemy phase build because the later only restricts enemies movement on player phase, which Leo is less suited for. The +Eff refine allows for leo to destroy common arena units, notably brave Lyn, who can't double Leo due to Leo having much higher defense. Slightly reduced damage from magic attacks also help.

Moonbow or glacies works well for Leo honestly. Reposition is also always good.

Distant def with a super boon in defense allows for Leo to reach (35/36) on enemy phase which is ridiculous for a cavalier mage. QR is essential to alleviate Leo's poor speed stat. Even def wave can give Leo a massive 41 defense on enemy phase, also the skill can be foddered from Silas. Res ploy improve's Leo's damage output but is more scarce. Fierce stance grants Leo a respectable 49 attack on enemy phase without cavalry buffs.

Brynhildr's effect applies to only ranged units, and unless using Brash Assault and leaving Leo at 17 - 20 HP (whatever your (<=)50% mark is at), the only way he can double all but their slowest is with the use of Quick Riposte, which only works on Enemy Phase.
This pigeon holes you to using Distant Defense, which is already available as a Sacred Seal, AND recently became a lot less expensive as an A skill with the introduction of Garon and even more so with the addition of Hero Grails.
Aether is equally priced, if not a little less expensive, being available on Chrom, who can be pulled at 3-4 stars unless he's featured on the Banner in use, and unlocking as an inheritable skill at 5* rarity, but when it activates, it compensates for his low Atk by reducing their Res to 50%, and it gives him self sustain, something he really needs given that Quick Riposte only works (<=) 70% HP, and even has the advantage over healers in healing him Enemy Phase, potentially allowing him to take on more foes in the same round without falling out of QR.
His assist could be Reposition for sure, as it works well on Fliers and Cavalry, but Drawback is a less expensive skill if you don't have enough Reposition Fodder lying around, and does have some circumstances where it performs better. Arguably, Swap, Pivot, and even Smite all can work as they would be placing Leo in harms way (Enemy Phase) while moving his ally into a (preferably?) safer spot.
His C Slot could be one of the Cavalry buffs, all depending on your team composition and play style, but you could play around with the Odd/Even Wave and Tactic skills for mixed team composition.

An in depth look at his IV's:

Boon:
*Defense is his best by far (in this build), not only as it's a Super Boon, but as it boosts refined Brynhildr's effect. This is preferred, regardless of whatever Bane accompanies it.
*Any other boon, although maybe usable to some extent, does not achieve much with the statline he is given.

Bane:
*That said, if you do have a choice, HP is your best Bane.
*Leo needs every scrap of Attack he gets for his already poor 43, 40 with a bane (and 46 really isn't worth a boon), so avoid this bane if possible.
*Res, Spd, and HP are all very usable as Banes.
*His Res does come into play when baiting Mages and Healers, and is a super Bane, but with Distant Def 6 (Effectively), that would still sit at a fairly high 38 Res, 44 when Fortify Cavalry is in effect, and that is on top of the 30% damage reduction from all magic foes first attack.
*Spd is questionable, it definitely loses all use in enemy phase assuming Brynhildr's effect and Quick Riposte are both active, but if ever you need to use your Leo in player phase (something that will inevitably happen if being controlled by the AI), although 28 Spd (with Hone Buffs) isn't going to double anything above 23 Spd, which still increases if he happens to be a bonus unit, it prevents followups from anything at or below 33 Spd, which covers a lot of units that normally utilize Distant Counter, so although it's definitely usable, taking a Bane here would almost definitely limit him to Enemy Phase only.
*Although HP takes away from both his magic and physical bulk, he has a fair amount of both already, especially with Distant Counter 6, and Brynhildr's refined effect, so this is probably the Bane that takes the least away from Leo's function.

Being a Leo enthusiast, I've taken the time to experiment with all kinds of builds for him. This is just the one that has worked the best for me with his new Brynhildr refinement.

The real star of the show here is Atk/Def Bond. Given the skill is so rare, you got to have a real love for Leo to consider foddering it to him. If you don't want to give him this skill, you could also go for Atk+3, Def+3, or heck, Distant Defense would also be really reasonable.

That being said-

With Brynhildr's amazing refinement, Leo can easily tank many ranged threats that come his way. Even blue mages to an extent. He may not be able to KO on retaliation, but he will for sure live most of the time. At times, this can open up opportunities for teammates with Wings of Mercy. You may even score some defense wins and Arena with it.

Now you may be wondering why I have put Blazing Light down instead of the typical Iceberg or Moonbow. Let me explain.

Blazing Light on Leo in particular can be quite devastating. You won't be hacking away through the weapon triangle to kill blues like you would with other specials, but you will be able to cover greens and reds very well. The main benefits to Blazing Light are scoring 2HKO's that Leo would otherwise not get with a normal special (especially against fast swords, which he wouldn't double without Swordbreaker), and being able to 'carry it around'.

Meaning if Leo happens to tank one ranged foe, and doesn't need Blazing Light to finish said foe off, he can save it for the next one he encounters.

Unleashing Blazing Light also synergizes well with Savage Blow and Speed Smoke. Anyone caught in the blast radius of it will sustain the damage from Blazing Light *as well as* 7 chip damage and a -7 speed debuff. Makes it nice for team follow-ups.

So yeah, it's fun.

If you find yourself facing more DC reds than you're comfortable with, you can also opt for swapping Speed Smoke for Hardy Bearing to get Blazing Light off safely.

Brynhildr (+Eff)
+Def/-Spd

Brash Assault 3 Seal

Bait favorable trades to enable brash assault, then utilize Leo as a glass cannon unit that can suffer a poke from other magical units. Ideally Leo should be supported by an ally with reposition to allow him to safely make the first poke and any defense buffs are appreciated.

Brynhildr's new +eff refinery grants Leo an Earth Sweep effect similar to Myrrh's Great breath, but the condition is that Leo only needs to have a point higher in defense vs ranged foes. Def smoke 3 allows Leo to cripple the defense stat of his enemies that surround his target within 2 spaces and amplifies his chances of procing his unique effect from his tome.