Finn

As a Tempest Trial unit, Finn really suffers for not having any IVs and merges. Also, his statline is very similar to another neutral unit, Camus.
That being said, even though that, he stil is a very solid unit who performs very well with investiment.

Weapon: Firesweep Lance+. The 15 might of this weapon turns his 34 attack stat into a respectable 49, along with LnD3 Finn is able to hit 54 for his attack. He also has enough speed to double most units, besides the most recent units, and not be hit back.
Assist: Reposition. The best movement assist in the game.
Special: Moonbow. Standart, quick to charge special that deals a lot of damage. Finn hits hard with this.
A skill: Life and Death. Finn doesn't come with the best bulk of all, so dropping it even lower to get his offensive stats up isn't a bad idea.
B skill: Drag Back. If by chance an enemy doesn't die by his lance, Finn will drag the enemy back, away from his team safety and let his allies finish them off.
C skill: Goad Cavalry. He is an Horse Emblem team, so his friends appreciate the buffs. He also can run any tactic skill, in mixed teams.
Seal: Initiate Seal Atk 3. The more attack the better!

This is a high investment Enemy Phase build. The stats shown are his bases with the shown skills equipped and him not being merged +1.

Tannenboom! stacks with Kestrel Stance and Close Defense for a stat spread of up to: 54 Atk/39 Spd (40 merged)/44 Def/26 Res on Enemy Phase against melee units (not including the buffs from *Def/Res Link 3).

There are obvious reasons for having a high Atk. Having 39 Spd lets him take less damage due to not being doubled as often, while also allowing him to double more enemies. His incredible Def (up to 50 on Enemy Phase) will make him take very little physical damage, even from a lot Axe Units (although his returning strikes to these Axe Units won't hit quite as hard, so a Bonfire proc is needed if you plan to use Finn to deal with them). He also will hit for up to 25 additional damage upon proccing Bonfire. His Res is usually very neglegible, since you aren't using him to tank mages (Green Mages [and some Blue ones] will oftentimes destroy him, so try to keep him out of range as much as possible). An exception to this are Red and Blue Dragons, which will activate Close Defense for 26 Res (32 with Def/Res Link buffs). I would recommend being on a Defense Tile if you plan to do this (unless none are available or you have the buffs from Def/Res Link).

He works wonders with a Ward Cavalry user. You can stack hidden combat bonuses to help with his tanking abilities this way.

Since he can apply his own buffs and apply buffs to other units at almost any time during player phase, you can use Reposition much more supportively. Some examples are moving another tank in range of an emeny (or having an ally do this to Finn) or moving a Bladetome user and having them nuke (while also allowing easier access for them to get into Desperation range). You can also use Reposition much like you normally would and have Finn move an ally out of range while moving himself in range.

Since none of his equipped skills are locked behind an HP threshold, so you can feel free to heal him back to full health without worry.

*I only put Def/Res Link 2 on the skill list because Def/Res Link 3 isn't available to choose from for some reason.

Just a random set I thought that would work. If you don't want to use brash assault, another idea I have is heavy blade, and you can make it work with Cavalry buffs, specifically hone cavalry, to make use of heavy blade to get an early special.

As you can expect, IS didn't put much effort un this character, giving him really mediocre skills....

I propose this build making a full offensive Finn with awesome 61 ATK (With Earth Blessing and Legendary Ike), and 41 SpD. Also, if you use other Cavalry characters with skills like Hone Cavalry Finn will have 67 ATK and 47 SPD that is incredibly high stats!

#MakeFinnGreatAgain

Weapon: Berkut's Lance+ (+Res)

Assist: Reposition

Special: Aether / Bonfire

Passive A: Close Defense 3 / Distant Defense 3 / Distant Counter

Passive B: Quick Riposte 3

Passive C: Atk Smoke 3 / Def Smoke 3 / Savage Blow 3

Sacred Seal: Distant Def 3 / Close Def 3

Finn has a rather balanced stat line that allows for some flexibility. This build specifically is focused on enemy phase tanking.

Finn's base resistance stat of 18 is unimpressive. However, Berkut's Lance+ (+Res) grants a whopping +11 res when the enemy initiates the attack. This turns his res into a usable 29. Giving him Close Defense 3 for an A skill will boost his defensive stats further to 38/35. Using Distant Def 3 for his Sacred Seal will allow him to enjoy boosted defense stats no matter what range the enemy attacks him from. The same applies if Distant Defense 3 is used as the A skill and Close Def 3 is used as the Sacred Seal. Alternatively, Distant Counter can be used in order to strike back against ranged units instead of just tanking the hits, though this is a more expensive option and does not provide any buffs to his stats.

Quick Riposte 3 makes it easier for Finn to charge his special and score more kills so long as he stays above 70% HP. Aether has a long charge time, but with Quick Riposte 3 it will activate more often. The healing also helps to keep Finn in Quick Riposte 3 range. Bonfire is a cheaper option for his special that will charge faster and benefit greatly from all of his defense buffs.

Finnally, the C skill is flexible. Atk Smoke 3 increases his tanking abilities by reducing the amount of damage he will take, while Def Smoke 3 allows him and his allies to score kills more easily. Savage Blow 3 is the cheapest option and lets him weaken nearby foes during the player phase.