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.

Posumudi + Meposumudi used on the first turn will reduce poison infliction chance by 60%, adding Seiran will bring it to 75%. Thus potentially shutting down the the probability of getting poisoned
- Mad Gasser AI may be affected by this (further testing needed but I never saw him use Mahapoisma with this strategy)

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.