Surt (February 2020)

Overview

Surt will not cast Ragnarok+ if you have Rama as your Anti-Pierce and two Drain Fire demons alive, instead choosing to spam God's Hand. His accuracy is pretty bad thanks to low AGI, so with buffs and some luck you can dodge those pretty easily.
Forneus AI seems to randomly choose a buff/debuff skill to spam each turn. He also carries Recarmdra so he should be the first one you focus down.
- Once Forneus is dealt with, the Boss will have no way of removing buffs/debuffs. With Tarunda and Rakukaja active, God's Hand will do a measly ~400 damage, which your Healer should be capable of cleaning up.
Tsukuyomi AI is also random, he will blindly AOE into Anti-Pierce, burning press turns. As such, it might be beneficial to keep him alive
- Shiva A is incredible for this fight, as he absorbs both Fire and Force in Teal when paired with Rama.
General strat: Bring an Anti-Pierce combo, set up buffs and debuffs and focus down Forneus.
- Lydia is great for extended fights like these, as it has no usage limits.

Encounter

Wave 1:

Power	Fomorian

Wave 2:

Tsukuyomi	Surt	Forneus

DPS	DPS	Anti-Pierce	Buff Control
Shiva A	Mahakala	Rama	Amaterasu
Butcher	Fire Boost	Fog Breath	Recovery Boost
	Back Attack		Samrecarm

Seiran is the preferred Dx2 as she will provide random healing and increase healing effectiveness. Templar Dragon can also be used to buff Mahakala's Concentrate.
Shiva A completely walls Tsukuyomi, killing off 2 press turns each turn. Third Eye also provides some decent chip damage.
Mahakala has strong piercing fire DPS which also drops the boss's defense, cancelling out any Defense boosts from Forneus. He also gets free AOE damage every 3 turns, plus concentrate to let him nuke even harder.
Rama is your Anti-Pierce unit. Fog Breath helps to lower Surt's accuracy and damage.
- Mabufudyne is nice to generate extra press turns off Surt. Yellow can also be used to alleviate some of the buffing pressure off Amaterasu.
Amaterasu is used for buffing and healing. Samarecarm transfer in case you get RNG screwed early on.
Once you kill off Forneus and set up buffs with Amaterasu, you have pretty much won the fight. Keep buffs up, ignore Tsukuyomi and just focus down Surt.

DPS	Anti-Pierce	DPS, Debuffs	Buffs, Healing
Frost Ace	Alilat	Seth	Amaterasu
Great Aim	Tetrakarn	Phys Boost	Resist Elec
Assassin	Lydia	Savage Glee	Samrecarm

I used Chalk Eater for this fight for some extra buffs, MP, and press turns. Frost Ace constantly hits Surt's ice weakness, while you can have Alilat and Amaterasu basic attack Tsukuyomi instead of passing.
If you have Tarunda and Rakukaja up, Surt will basic attack into Tetrakarn and waste press turns while doing no damage.
Kill priority was Tsukuyomi -> Forneus -> Surt, as Surt does no damage if you can get him to consistently attack into Tetrakarn.
Keeping Tetrakarn, Tarunda and Rakukaja up was the most important part of this strategy. Seth's Debilitate was needed occasionally to ensure that Tarunda stayed up.

DPS	Dodge Tank	Anti-Pierce	Buff Control & Healer
Zaou-Gongen	Garuda	Alilat	Amaterasu
Epitome of Carnage	Evade	War Cry	Samrecarm
Savage Glee	Speedster	Lydia

Pure fusuable team. P3 Zaou is must for the crit + Phys Pierce.
Used Eileen for extra HP and Lucky Skip.
Alilat + 3 Fire Repels to force Surt to use God's Hand which has no pierce.
As Surt will target the demon with the lowest HP, make sure Garuda has the least HP and stack dodge brands and Phys EV% to dodge God's Hand as often as possible.
Garuda can use Fog Breath to lower Surt's accuracy to dodge more.
Use Alilat for Lydia and pass to cycle through for passive HP recovery.
Zaou is the main damage dealer, make sure Tsukuyomi is dead first so Garuda won't die to his Magic.
Amaterasu is to undo the debuff of Cry skills from Forneus plus she provides healing if needed.
Once the adds are dead, Just keep Buffs & Debuffs and Lydia up, pass for MP and use God's Hand or regular attack with Zaou to slowly whittle Surt down. (Zaou will deal more damage than Surt can heal from Repelled Ragnarok should he cast it)
Make sure Zaou gets the crit loop going during the prelim and pass through for full MP recovery before defeating the prelim wave.
Use Auto-Rebellion on Zaou if you have it to make the run smoother.
Very time consuming, do not try unless you have 15 min or more on your hands for 1 boss fight lol.