So first off I'll start with the red sword
Eldigan +Spd/-Def
Fury 3
Desperation 3
Threaten Spd 3/Any Threaten skill
Mystletainn/Brave Sword +
Reciprocal Aid/Ardent Sacrifice
Moonbow
This basically makes Eldigan a superpowered sword nuke capable of taking out any squishy hero in the game with ease, especially since Eldigan has a huge base stat total, it certainly helps. I prefer Threaten speed to help him get doubled less and potentially set up other units for doubles, though any skill that works with a given team is fine. I'm not sure whether Brave Sword+ Or his special weapon is better, primarily because one turn Moonbow is insane, at the same time Brave sword works wonders with Desparation 3 and Fury, and it's still practically a one turn Moonbow. At the same time, the minus speed has me a bit worried, so I'm still on the fence. Technically the support skill can be anything, ardent sacrifice can set up into desperation, but I think reciprocal aid could also help if he needs healing bad.
Onto the Blue
Sully(Mine is +Spd/-Atk)
Sapphire Lance +/Brave Lance +
Draw Back
Death Blow 3/Any other "Blow" Skill
Swordbreaker 3
Fortify Calvary
Sacred Cowl
Sully is meant to deal with sword units, make Eldigan a survivable Nuke, and provide tactical possibilities with draw back. I also choose Sully over Abel due to the fact she has higher Spd, especially with a +Spd Iv, she has the potential to double on non-sword units. Now, in terms of brave vs Sapphire, I figure offensively I can have Eldigan sweep through any greens that dare get near Sully, including maybe even Hector (didn't check the math on that though), so I believe pushing her niche as the sword killer is more worth it then having a brave weapon. I'm gonna use Death blow primarily because my Sully is -Atk, but feel free to use whatever you want. Sacred Cowl to help her survive ranged attacks, maybe one from a Julia or Nino. Fortify Calvary is basically there to make sure Eldigan is around to nuke their team. And you know, help out other units too.
And now In the green corner
Fredrick +Atk/-Res
Brave Axe+
Harsh Command/Really any other support ability
Escutcheon/Luna
Death Blow/ Anything else Really
Lancebreaker 3
Hone Calvary
Before you ask, yes I know Gunter has two of those skills, Fredrick just has better overall stats, besides that, my Fredrick has better IVs then my Gunter. Anyway, I was debating between Hammer and Brave axe for Fredrick, simply because Effie eats Eldigan for breakfast even when superbuffed, however Brave axe plus Lancebreak gets the job done pretty well I'd hope (Again, haven't done any math) plus a death Blow and Fredrick's already surprisingly good Atk should OHKO any blue lancer. Escutcheon could be good if I'm worried about Fredrick surviving, but Luna could work potentially well with the brace axe. I could probably give Fredrick Moonbow as well, but eh, that's more Pallas then I can afford, but that's probably a better option than Luna. I have Harsh Command just in case Eldigan got debuffed along his killing spree. Hone Calvary is the bread and butter of this whole team, +6 to Atk and Spd is ridiculously good, especially on Eldigan, who is also getting boosted with Fortify Calvary, plus Eldigan's Fury adding an additional +3, and all of the sudden that's a +9 Eldigan in all stats, adding to his already impressive stats, running around the battlefield with a one turn Moonbow that's auto following up with Desperation, not to mention the enemy most probably cant double at all since Threaten Spd is a thing.
As for the fourth slot...
Tbh you could probably stick any other Calvary in here and it would probably be ok. I would recommend an Elise. Not entirely sure what the best skills for her would be, so that's something to discuss. As for me...
I filled my fourth slot with Ursula
Blarraven+/Blarblade+
Reposition
Death Blow 3
Bowbreaker/Renewal
Savage Blow/Breath of Life/ Anythreaten skill
Iceberg
Ursula is primarily here because #BlameTakumi, but also because she is a Calvary and synergizes well with this team. I'm on the fence with Blarblade and Blarraven, since Kagero, Takumi, nad Jaffar are all really good. However with Blarblade, Ursula can benefit from the super buffs as well, maybe even more than Eldigan. It's something to think about though. Reposition is going to be primarily used to help move other Calvary units to get adjacent to each other, Death Blow is still nice to have on her, Bowbreaker because screw Takumi, it's either that or Renewal to have a bit of artificial healing if I give Eldigan Reciprocal Aid, and keep my Takumi killer alive. Savage Blow is probably unnecessary since I expect the rest of this team to OHKO most heroes, so the chip damage doesn't matter as much, but guaranteed damage is always good. Breath of life is probably the best choice since there are no other healers on my team. Threaten skills can work on her, though probably not as often since she won't be in front as much as others. Ursula has a respectable Res stat, so Iceberg works well. Not glacies primarily because she probably won't double that often unless you give Darting Blow instead of Death Blow.
TLDR; Superbuff Eldigan to infinity, go ham, have everyone else back him up if needed.
Any thoughts and opinions, maybe other suggestions for units or skills?