Warning: This highly relies on chance. At any point, it can fail. Proceed with caution.

What You will Need

Required:

• A copious amount of luck
• Echelon 4 or 5
• 7* Vossler capable of 350% elemental resist for lightning/wind/earth/dark after buffs (can be reduced with absorb buffs, notably Karten's)
• AoE reraise OR A provoker that isn't Vossler w/ 100% ele resist after buffs OR
• ST Reraise if not using AoE variant (edit: redundant)
• A source of self-dispel
• Damage and morale filling
• Status Ailment resistance buffs

Optional:

• AoE Mirage
• Even more luck

What to Do

Turn 1:

1. Dispel any DoT from previous turn.
2. Use Vossler's CD phys cover
3. (Optional) Apply any mirage stacks.
4. Pray.

Turn 2:

1. (Optional) More mirage stacks
2. Pray.

Turn 3:

1. Use any of Vossler's magic cover.
2. Apply elemental resist buffs.
3. (Optional) More mirage stacks.
4. Pray some more.

Turn 4 (Provoker Route)

1. Apply Reraise on Vossler
2. Provoke with provoker
3. (Optional) Apply mirage.
4. Pray.

Turn 4 (AoE Reraise Route)

1. Apply AoE reraise.
2. (Optional) Apply mirage.
3. Pray.

Why Does This Work

The magic modes is fairly standard; gear for elemental resistance and reraise for the one non-elemental one. Mirage is to mitigate casualties from the rare unlucky normal attack.

The physical modes is reliant on Vossler's meme physical cover:

Sword Parrying

• [4 turns cooldown, available on turn 1]
  
• Chance to protect all allies from physical damage (7%) with damage mitigation (100%) for 2 turns to caster

This ability reduces all physical attacks to laughably low damage with no relevant survival gear against any physical attack. The cooldown and duration is also exactly long enough to make it maintainable for the entire fight, if the team lives long enough...

The catch is the ridiculously low cover rate. The good news is that the hydra spams its AoE attack at the start of T1/2 at E4/5, and cover chance is calculated for each other party member. This leads to a 30% chance of survival on its own. This chance can be increased with AoE mirage to give more buffer against bad RNG, leading to 66% survival rate with 2-stack mirage, and 89% for 5-stack mirage.

Yes, this also means that, theoretically, a clear is possible without Cressnick or Sieghard, or clearing the fight ultrafast. It's fairly gear heavy for the elemental resist though.

That said, due to the high risk involved of failure, I cannot advise this barring a last resort for clears. More a proof of concept and fun little thing to try out.

Special thanks to ReidKu on discord for testing the initial concept and providing the images.

Edit: Just noticed that its final attacks also ignore provoke. And added ReidKu clear image proof.