Surtr counter

Surtr Buffs what are those, the only time she doesn't kill is if he's running Wary Fighter

axebreaker sanaki does the job for me,
and a buffed up tharja works most of the time as well if you dont have a sanaki

A triangle adept, axe-breaker, armorsmasher unit should hard wall him. Units like Palla and Alphonse with their TA refines have some use here. I'm not sure how you go about doing Aether Raids, but I have different teams with slight variances to deal with the various BS in this game mode. My Surtr counters are Axebreaker Aversa and TA Laevatein.

Surtr is one of the worst Units FE ever made.

But still is free win if you get a 5man Team Surtr what is to my suprise actually common.
Get any redsword with TA example (be care to slow red Sword may get SPD check vs Wary Fighter) rdy for backup Team put him Axebreaker and he murder Surtr Teams no Problem add a healer cause of 20 dmg per tick.

But Overall Aversa Axebreaker clears most in time vs Surtr enemy Team and Aversa debuffs help more in AR.

Aversa with readied moonbow. Axebreaker works too but I like using her B slot for chill effects, and is a moot point if he isnt running wary fighter (a lot dont). Whats fun about chills is that if multiple units share the highest targeted stat ALL of them get the debuff. So that could be helpful against surtr teams if you have access to chill res (unless some of them have boons/banes or merges or whatever)

For groups you could try running an aoe special on a ranged unit. Ophelia is pretty effective against him, despite her color disadvantage (which the special ignores). Could also use dazzling pain+ and savage blows to chip them down.

He's only stupid right now because hes a bonus unit. Without that +10 hp/all stats he eats dirt against most any red mage.

I've said this before but if "null no counter" or whatever is inheritable by armor units *that* will break the game by putting them over the top. Right now the only thing keeping armor units in check is the fact that they can be chipped down by sweepers/dazzling. Take that away and the only way to beat an armor team is by running stronger armor units ...

My BLACK KNIGHT being the BOLD FIGHTER that he's gives this green armored guy a DEATH BLOW with his BRAVE SWORD.
Sometimes this green fellow is so scared that I can easily THREATEN his DEFENCE, but given that my knight is really slow I bought him some ARMORED BOOTS so he can run faster.
He's at his strongest when the BLACK LUNA shines upon his magical armor.

Bold Fighter Hammer Sheena.
EZ.

Laegjarn is a hard counter to Surtr, i run her last week in arena and was a peace of cake to get rid of him.

Tobin was free.

Refine Armorslayer to -smasher, then give that the Def refine or the Null Armor refine.

Give him Axebreaker from 4 star Laslow.

Give him Triangle Adept from a 4 star Roy.

Boom, instant free Surtr counter.

I use axebreaker Lilina. Surtr has never been a problem for me

Hana with her base kit one-shots a base surtr (you may need an attack boon, but that should be really easy to get).
Also, if you give her vantage and the brazen atk/res seal, she can either kill him offensively, or bait multiple surtrs and one-shot them on the counter. Probably the cheapest surtr counter I can think of, you can give her axebreaker if you want to be doubly sure of killing