Leo

Leo was my 2nd 5* that I actually used. Laugh all you want, it worked for me XD
The Gravity effect with his bulkish stat-line really saved my ass on some maps.
he is obviously been completely outclassed since, but:
Brynhildrs because it is cheap, Repo because Cavalry, Growing Light because I always preferred bigger range, even though for him it still makes no sense.
Death Blow 2 was because I didn't have the fodder for DB3, and I wanted him to have some player-phase power, QR3 is there for the moments he was thrown in to tank, and Savage Blow to chip the other units and because it's cheap.

well easy. Stack defense and destroy with counters. You will need lots of def. since the speed meta is quite out of his league. C could also be threat. atk def 3 but res does give you a little extra umpf at no merges and with good positioning skills leo will get + 16 def so 45 def on a non merged unit. If you really love leo You can give him summoner support for extra def also flowers 48 def if this is your deal.

You can also use Gunter for more def. or ward cavalry on any ally. drives would work fine too just....Don't make him fight a mage or a manakete at close quarters

Btw ruptured sky will work wonders too

This is my pride and joy - a +10 OG Leo that I use in Arena. I decided to take advantage of the fact that Leo's refine allows him to deny a lot of prominent ranged units that are easily mergeable, such as Nino and Soren. He also has the added bonus of having enough mixed bulk and bulk, in general, to challenge most green dragons and some sword units without much trouble.

Why did I choose to +10 Leo? I mean, I just like him a lot as a character. That's all.

+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.