Finn

The product of extremely good luck in the Hall of Forms, the set is fairly straightforward. With cavalry or tactics support (as always adjust the C skill accordingly) Finn reaches 68/52 offenses at +10 with full dragonflowers, which is generally enough to proc heavy blade and attack most targets twice, with the second attack activating Ruptured Sky. Combine this with Lull Spd/Def and you have a monster that will eat most targets alive. If still concerned about units with higher attack/speed/defense ruining your day, adding chill/sabotage/ploy/Aversa support nullifies most anything else.

Note: Like most units, does not work particularly well against Null C Disrupt Brave Ike.

A fast, hard hitting tank...
Deft Harpoon offers additional atk and spd alongside atk spd solo to bring his spd up to a respectable 41 spd and 54 atk. This will help him double and take down most units.

The def+ Deft Harpoon coupled with Close Def 3 gives him 42 (44 if health is full) defense making it extremely difficult to damage him, through physical attacks. Along with the passive healing from mystic boost, Finn will pretty much always have full health, allowing Deft Harpoon's weapon effect to remain active.

Def Smoke debuff the enemy, increasing Finn's damage output on surrounding enemies.

Moonbow was chosen for it's fast cool down however, you can use ignis instead if you want to channel his sky high defenses into offensive power.

I held a vote in a few Discord servers I'm in on which unit I should build, and out of 15 (including a perfect IV Hana), Finn came out on top. Since I don't have the money to build a completely optimal Finn, and since orbs have become increasingly harder to come by, I had to get a little creative with this build.

Brave Lance+ is chosen as the weapon of choice, as it's the weapon Finn comes equipped with. It works decently with his stat spread, but it can become a weapon of mass destruction if utilized properly.

Finn's assist is Reposition. As a unit who can move 3 spaces in one turn, he can be used to go across the map and save a unit from being wiped out.

Moonbow is the special of choice here. Usually, I'd opt for Bonfire or a similar skill, but Finn's Defense isn't high enough to get the most mileage out of Bonfire. It works well with his Brave Lance.

Life and Death seems like a strange choice for his A passive, and I admit, it is. I didn't have a lot of options, seeing as he is currently third on the waiting list for Klein (I have a perfect IV LA!Roy and Lilina). Life and Death buffs Finn's Attack by 5, while also negating the Brave Lance's penalty. 27 Defense is still acceptable for taking physical hits, but 13 Resistance means that unless he's supported by another unit (particularly a dancer), he should avoid any sort of magical encounters.

Lancebreaker was chosen above Swordbreaker due to the weapon triangle already giving the Lance of Legend a hefty advantage against red foes. However, if you choose to use this build, swapping it for Desperation is also something to be considered.

His C passive is flexible. Since I only run mixed teams, I went with Drive Spd. Another suitable option is his native Goad Cavalry, or even a Wave skill if you have a 5 star (or 4 star Aversa for Odd Res Wave) you're willing to give up.

Atk/Spd 2 was chosen for the seal due to the offensive buff it supplies him. Alternatively, you could use Darting Blow, one of the Brazens, Quickened Pulse, or another stat buff.

Weapon: Wagasa+(Spd refine)
Assist: Reposition
Special: Glimmer
A: Swift Sparrow 2
B: Chill Spd 3
C: Atk Tactic 3/Any Tactic skill
Seal: Spd +3

Build made to turn Finn into an offensive powerhouse. This build makes him very formidable in normal combat, as when he initiates he will have 43 Speed and 52 Attack. He also has a decent 32 Defence, so assuming you are fighting physical targets, he will most likely survive, and assuming he doubles, he will be able to proc Glimmer. Glimmer is chosen over Moonbow as the chill def effect of Wagasa+ means that Glimmer will be stronger in that situation.

He is also very flexible in any team composition, as he provides a crippling debuff to one or two enemies, and being a cavalry unit, has access to goad/ward cavalry.

Finn with that build has an incredible melee defense although the speed is not for this build.
Without much effort the armored ax fall quickly in Enemy Phase mode as well as all of Hectors and this is because they can not double for their speed of 35 and their incredible defense with those abilities.
The C gives a good attack if the target survives anyway will die.
And the special with all that defense ensures a fast finish as Lukas says.