Can't believe I'm asking this

Firesweep+Hit and Run+Poison Strike Seal into positional skill.

You can run some calcs to see if there's an axebreaker build that can OHKO him.

Just have this girl on one of your team one shoted my surter...

Any green or red armour running BF with something like Luna works.

Palla now I guess can do it? Never count Tobin out. If you buff EST enough she might because her refine is nasty

He's incredibly annoying because you can't bait him, he makes the trap backfire, he eat your bait and drink your milkshake

He'd be a lot easier to handle if Null Follow-up was actually readily available (assuming they actually care about balance, it's pretty unIntelligent Systems to make a pivotal anti-meta skill so rare). Otherwise, he may force the generic red unit to run Axebreaker.

But Caeda and Palla can do damage against him with their PRFs/refines (I presume).

Alm and B!Celica can maybe do due damage as well.

Laegjarn, bait him out take the 20 from Sinmara and the debuffs - His buffs counteract them anyway, may need to "bait" a second time, then go for the kill.

Vantage Ares with Luna. As long as his special is charged, he can run in and smash Surtr hard with Luna. Then on enemy phase, Sinmara activates Vantage and Ares smacks him with Luna again. This kills the crab.

A dancer also works if you don't want Ares taking the Sinmara hit.

Kite him with dancers, chip him, use axebreaker, hit his res, pre-charged specials

You might also like to search up a certain question Vergessen asked not long ago about reds who can 1v1 this defilement unsupported. Zelgius, Hinata, and Ogma were on the list. Fir could do it with minimal support if she has max merge, even if Surtr also has.

Est can actually kill him with the prf brave effect if both her nearby flying allies use goads. 38 atk + 19 from weapon + 6 from death blow + 8 from goads = 71, which is 81 against a green armour without Svalinn shield. Even if he has +def and steady stance 4 that's only 55 def. 81 minus 55 is 26, and 26 twice is 52, which is more than he has HP.