Surtr can't take a Speed or HP bane if you want to hit 180 BST.

I feel like Def. His weapon already gives him +3 Def.

Trick question. He should go -Atk +Def. He already has more than enough attack and his main purpose is to sit there and take attacks while Sinmara does all the work.

his defense is already insanely high so -def is probably the best.

Idgaf about BST. I'll take that Speed Bane, thank you.

Since I love my units to be as unkillable as possible, -atk +def would be good for scoring.

From a more regular gameplay perspective, Surtr is pretty IV proof.

Poor Zephiel... He's nothing compared to the fire giant.
I can't get over that 33 res.

Cripple my 15 Spd or my 31 Res? Jee, man. I don’t know.

If you actually care about having 180 BST, obviously it's an arena setup, so of course -atk. You don't want some enemy accidentally impaling itself on him instead of on the bonus unit.