Walhart

NOTE: Stats are at +10 merge with 10 Dragonflowers.

Walhart: The Conqueror. A severely underrated axe cavalry with the potential to be a deceptively fast and physically bulky. A true monster sweeper in PvE and can hold his own against plenty of high-budget arena builds, doubling most units in the game, and avoiding doubles from all but the fastest.

This Walhart aims to take advantage of Wolf Berg by supplementing his already impressive Atk stat, his physical bulk, and investing heavily in his Spd in order to make a powerful Galeforcer. Heavy Blade 4 provides extra damage while ensuring a GF proc on any unit he can double that can counterattack (bar those with guard). Lull Spd/Def helps to secure doubles on units with physical buffs, and the combination of Rouse Atk/Def, Atk/Spd Solo, and Wolf Berg ensures he's rewarded generously for being alone.

Assuming Walhart is away from his allies, he can reach an effective:
78 Atk
49 Spd
55 Def
26 Res

These stats also mean that he can tank physical hits on the enemy phase to help proc Galeforce on the next turn. All he's missing is a refine. IS i'm begging you please

This build requires a bit of setup, but considering the lack of easy to acquire green cavalry units it is definitely a viable alternative, especially if you like Walhart as a character.
Without merges Walhart hits 37 speed with Wolfberg and Atk/Spd solo active which doesn't sound like a lot, but there is potential for higher numbers through buffs before he rides solo into battle as well as having another ally with Chill Speed in their B or seal slot to even out the odds.
With Hone Cavalry active, Chill Speed and then Speed Smoke on the enemies who are (hopefully) surrounding the target who was hit by said Chill skill, Walhart can hit an effective 50 speed on top of his 69 attack. This pretty much guarantees a follow-up attack that triggers Heavy Blade and subsequently Galeforce if the enemy counterattacks, although enemies who ignore speed to avoid being doubled can be tricky to deal with. With merges and dragonflowers the potential goes up even more with frankly overkill attack that can one-shot enemies if you aren't careful and some extra defensive stats for when you need to safely get into desperation range. If you want a slightly more dual-phase build you can replace Desperation with Vantage to gain an extra charge to your Galeforce on the enemy phase as well as a potential cleanup kill or two before your next turn. Walhart can take a few hits which should allow him to stay in the fray if asked to, but foes who target his resistance needs to be taken care of by your allies unless he can safely take them out himself before they get a chance to hit him.
The conqueror is a rather versatile unit and a bit underrated as far as cavalry units go, although the massive investment and terrible resistance might make people shy away from building him. That being said, he can put in some serious work and should at least be considered if you lack a green Galeforce sweeper.

Lvl. 40+10 Stats:

45 HP
57 ATK (68 with Atk/Spd solo and Wolf Berg active)
31 SPD (47 with Atk/Spd solo and Wolf Berg active)
43 DEF (47 with Wolf Berg active)
20 RES (24 with Wolf Berg active)

Just buff this with a fortify cavalry and send it alone to combat. As long is not speedy red mages or something effective against cavalry (albeit i admit these two are not so uncommon) it will survive almost anything, since after the first attack he will most likely get another +7 to res with the sacred seal, meaning if it was fortified and he is solo, it will reach 39 res! (No merges)
Assist skill is up to preference, for the special i put some healing since his attack is already skyrocketting and he also has chill def. It's important to be a small healing effect, this way he will stay inder the 80% hp threshold.
The c-skill add great survivability, since it will debuff attack of his enemies.
Another option is if he's not up to very speedy enemies, he can run brazen spd/res 3 in the A skill slot instead, this way after being damaged he will have 38 spd, also possible to be increased if it is sent "behind enemy lines" after being honed too, and even maxed if atk smoke is replaced with speed smoke, but i think this would be pushing it too far.

Walhart. Theres not much to say him except that he is a physical tank on wheels. While the fortress def takes away some atack he still is really strong. And the heavy blade sees to that.