which characters to make Surtr even more OP

3 Male corrins with refined yato. And a ton of drives or spurs.

You could use M!Corrin using a refined Yato, with a support and 2 drives...
Boy howdy.

Anything that helps him move around or heal is also welcome

- Units with hagoita, kadomatsu, and hama ya
- Healers, especially those with dance support
- Units with ward armour
- Legendary heroes (or mythics) who can give him more def or res
- Units good at dealing with reds

3-5 eirs, to patch his res, and heal 30-50 HP every turn for him.

Flying Olivia, running Guidance and switching out Chill Speed for either Chill Attack or Chill Defense. You could swap her weapon to Kadomatsu if you already have visible buffs on the team.

Any refresher, buffer, or healer honestly. Just don't be this guy.