Walhart

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.

Merge: +0
Seal: Speed +3
Blessing: Fire

Wolf Berg (Weapon)
Unlike in the original game, Wolf Berg in Heroes lacks the ranged effect and comes with a new effect entirely. Granting him Atk/Spd/Def/Res+4 when there are more enemies than allies near Walhart. This makes Walhart a pretty ridiculous solitary juggernaut if playing with the right cards.

Reposition (Assist)
Do I really need to go over this?

Bonfire / Ignis (Special)
He relies on Specials like Bonfire and Ignis since he gains a ton of buff towards his Defense stat. If he wants to hit harder, Ignis is ideal.

Distant Counter (A-Slot)
One of the most overrated skills in the game. This "restores" the ranged effect that Walhart's Wolf Berg lacks. Similarly to how Zephiel cannot attack from distance and gained a new effect for his Eckesachs.

But the main point is that Walhart can counterattack from any distance. So long it's not Brave Lyn or Brave Veronica or Razzle-Dazzle Staff units.

Vantage 3 (B-Slot)
The skill that will save his life from many life-threatening situations. He will easily crush many fragile Blue units at relative ease.

Hone Cavalry / Odd Atk Wave 3 (C-Slot)
Walhart prefers not to be in range with his allies, but if needed, he can provide Cavalry buffs if his Cavalry allies need it. Otherwise, he'll run a skill that fulfills his own needs.

Speed +3 (Sacred Seal)
Walhart isn't exactly slow, but he's not fast either. With the right setup, Walhart is surprisingly fast with someone with base 27 Speed. It's like what they always say: don't judge the book by its covers.

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Pros:
+ Very solid Attack stat along with Defense, making him a great physical duellist
+ Wolf Berg grants Walhart Atk/Spd/Def/Res+4 during combat while there are more enemies than allies in his range
+ Despite his base 27 Speed, Walhart is indeed slow, but because of his weapon Wolf Berg, access to Cavalry buffs, Skill Inheritance and Summoner Support, he can be surprisingly fast (of course, just about any units can achieve this through sheer dedication and favoritism)
+ He does not come with his usual Conquest skill, but still came with a skill that nullifies his Cavalry weakness whenever it is in need. That being Grani's Shield

Cons:
- Wolf Berg, sadly, does not come with a Distant Counter effect and must inherit said skill from any of the Hector variants that isn't Brave Hector
- Has abysmal base Resistance stat -- which he should be avoiding magic users in general, but... I don't think he cares
- Without Grani's Shield, Walhart is susceptible to Horseslaying weapons, therefore he takes even more punishment from Wolftomes due to his abysmal Resistance
- Wolf Berg can be rendered useless if he is too close to his allies to make full use of
- Following that, Walhart will not be able to receive consecutive Cavalry buffs each turn once he's gone off on his own
- His base kit resembles that from the original game, but it does him no good in most scenarios and requires Skill Inheritance... and if he wishes to be the strongest, he'll demand heavy and expensive inheritance

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.