Vasuki (March 2020)

Overview

Mad Gasser will only proc auto-Barrier Break at the start of the round. If you recast Barrier they have no way to remove it, almost trivializing the whole fight.
- Alternately, you can inherit Null Poison or bring demons immune to ailments like Kinmamon to alleviate pressure off your Barrier caster.
Both Vasuki and Hel have a passive that increases damage done to poisoned enemies, and Vasuki will cast an almighty-AOE when your demons are poisoned. If you used the approach above, you will never run into this scenario, severely reducing the entire team's damage output.
With proper buffs and debuffs up, you can mitigate the damage to a very manageable level.
- Hecatoncheires is a good choice for this fight, as he reduces AOE damage taken from both Hel and Vasuki, and can also double up as your debuffer / secondary Barrier caster.

Meposumudi used on the first turn will reduce poison infliction chance by 30%, adding Seiran will bring it to 45%. Thus reducing the probability of getting poisoned
- Mad Gasser AI may or many not be affected by this (further testing needed but AI may use Mahapoison less often)
- Buff/Debuff should be used parsimoniously. They defense buff or attack debuff seems to increase Mad Gasser's willingness to use Mahapoisma.

Encounter

Wave 1:

Blob	Black Ooze

Wave 2:

Hel	Vasuki	Mad Gasser

Barrier	DPS	Heal + Buff Control	Debuff + Barrier
Kinmamon	Yoshitsune	Amaterasu	Orcus
	Epitome of Carnage	Recovery Boost	Barrier
	Butcher	Samarecarm

I used Kangaroo Boxer for Position Hack, as the team did not have enough BS.
Kinmamon is immune to ailments and is just here to cast Barrier.
Yoshitsune is the main DPS demon. Mad Gasser has a passive that grants extra evasion though, sometimes it's better to pass than risk a Miss and lost press turns.
Amaterasu is used for buffing and healing. Samarecarm transfer in case you get RNG screwed early on.
Orcus provides a strong debuff in War Cry, AOE reduction and also acts as a secondary Barrier caster.
Save scum until Position Hack triggers and use the turn to set up Barriers and buffs. As long as you keep recasting Barriers and get some occasional Hassou Tobi casts in, you should be able to clear the fight without problems.

Repel Dark/Almighty	Repel Ice + Ailment cleaner	Heal + Buff Control	dps
Huang Long	Beiji-Weng	Alilat	Seth
Speedster	Meposumudi	Lydia

Seiran as D2
Five Elements cast at all time
Meposumudi on the first turn to increase poison resistance
- Possibly increase the probability of Mas Gasser of using Deathtouch which will get repeled by Five Element
Avoid using defensive buff or debuff on enemies' attack as it may increase the probability of Mas Gasser of using Mahapoisma
Alilat + Beiji (or any other Repel Ice demon) to prevent Halahala use, Hel will waste turn presses on it
Kill Hel and Mad Gasser first
- Vasuki AI will be reduced to Normal Attack only