What are the best ways to absolutely destroy Surtr.

Axebreaker Lilina.

Doesn't limit you to a 5* exclusive in any way, with enough firepower to ruin his life.

All u need is TA and/or axebreaker tbh. For melee units they should have 40 hp min.

For arena all i did was give mcorrin TA. He can chip him down sometimes but most times i do have to kill him off

If you still have your 5 star copy of him from a while back, I suggest Tobin. While he isn't the absolute best candidate for obliterating Surtr, he was free and doesn't need much to do it. Just refine Armorslayer to Smasher, then refine further to either the Null Armor effect or Defense. Give him Triangle Adept and Axebreaker, then give him whatever else you'd like.

Also, kill him when you have the chance, i didn't and i lost the entire match of AR for a single mistake.

Axebreaker on a high Atk red mage will do the trick. Lilina, Sanaki, Adrift Camilla, and Bridal Tharja can all reach very high Atk with their base skillset.

Armor effectiveness is another way. Most armorslayers will still need Axebreaker or Triangle Adept due to Surtr's absurd defense, though Soleil can pick up the OHKO with the right build. A high atk hammer can also work with Axebreaker. And of course, Dancing Micaiah just wrecks him.

Surtr's high defense doesn't protect from out-of-combat damage. Pain and Poison Strike builds can break Wary Fighter, which makes him much easier to deal with. A Surtr of your own can also help here. If you want to go deep, give Jaffar double poison strike for a clean 30 damage after combat.

Lastly, Ares with Vantage+Luna hard counters Surtr if his special is charged. Smack Surtr with Luna on player phase, then smack him again on enemy phase. This is far too much damage for Surtr to take, and Sinmara assures Vantage is active.

Micaiah or any other units with “Effective against armored”

axebreaker aversa with a dancer easily destroy him

Red Tomes, Red Dragons, Bulky Red Swords...Pretty much anything red really.

Adult Tiki can wreck his day too. Enough defense that she doesn't care about his attacks, and enough attack hitting on the res stat to make him go away for good. (QR recommended of course)

Best ways - use either of his daughters against him.

Soleil nuff said, and if you don't have wrath use axe breaker to bypass wary

Magic user (green/red) + AOE special. It can bypass wary fighter with the AOE damage and then strike twice for little effort killing.