How to deal with Surtr?

This is just off the top of my head, but Caeda may be able to take him down. She has triangle advantage and effectiveness, but also a Prf weapon and access to R Duel Fliers.

Armorsmasher Soleil probably.
If you can get units with Luna special ready, they'll do a lot of damage. (infantry pulse, ostian pulse, quickend pulse)

Bartre with Axe of Virility should do a good chunk of damage too

BF Zelgius or M!Grima with BF (Surtr has 17 speed at neutral... Unless you're -Spd Grima you should be fine). Most reds with Armour Smasher. Summer Micaiah would leave 0 trace of there ever being a body to begin with. Red Blade tomes or Red mage nukes (Lilina)

Brave Celica with or without armorsmasher can bring this guy down. She brings demise to +10 Sheenas with her royal sword, but she can attack safely with Firesweep or as a lunatic with a Wo Dao.

The point with Surtr is not just killing him, is attacking him in his comfort zone, this is the worst part since you cannot bait him, he will strike you with 20 dmg and generate an advantage of +8 stats over you, even Arden would have problems against that.

It will be better to use magic. Lilina and Celica can one shot him. Other stategy could be chipping damage to him. Lu Bu's six paths of pain+ build was useful for me during his map, since you can strike him with magic damage (hopefully with miracle active at this point) and in next turn receive your 20 damage to get in vantage range and kill him with close counter.

probably ideal to chip him with pain+ or Jaffar, especially by hitting a nearby target so that he gets hit with savage blow too

If you go back about two pages of questions from yours, you'll find that there was a lot of buzz about how to deal with Surtr, and you can look at the topics yourself. People were making suggestions such as anti-armour weaponry and powerful mages