Here is the full list of all 57 Hydra Slayer achievements.
Defeat the first boss with an Echidna.
Defeat the second boss with an Echidna.
Defeat the second boss as an Echidna, using at most one Potion of Life.
Defeat the second boss with the Twins.
Defeat the second boss as the Twins, using at most one Potion of Life.
Saved the Twin with a Potion of Life
Had one of the Twins return as a mirror image
Defeat the first boss with the Twins.
Defeat the second boss with a Centaur.
Defeat the second boss as a Centaur, using at most one Potion of Life.
Defeat the first boss with a Centaur.
Defeat the second boss with a Titan.
Defeat the second boss as a Titan, using at most one Potion of Life.
Defeat the first boss with a Titan.
Used the titanic artifact.
Defeat the second boss with an Atlantean.
Defeat the second boss with an Atlantean, using at most one Potion of Life.
Killed the first boss with weapons only
Win the small game without attacking with divisor blades
Win the small game without drinking your first Power Juice (not for Titan)
Reached and killed the first boss without drinking the first three Power Juices you find
Defeat the first boss with an Atlantean.
Clear the first level using no items or weapons except the ash scythe.
Commited a suicide
Tried to abuse the shields.
Was killed and came back to life (not as Twin)
Conquer 150 levels.
Made the Null Sector useful
Conquer 100 levels.
Found the crazy weapon and tried to attack with it.
Had 5+ active powders at once
Had 50+ mushroom seeds
Kill a hydra with a trap.
Killed a hydra >= 100 with the Silver Cutter
Killed a monster using the wand of phasing
Killed a 2000+ Hydra with an ambidextrous attack
Used the Mushroom Staff.
Created 200000+ zombie
Collapsed a Hydra under its own weight
Defeat the second boss with a Human.
Defeat the second boss with a Human, using at most one Potion of Life.
Defeat the second boss without using any Potions of Life, as any race.
Had a missile dropped under their feet
Slain a Hydra with at least 100 heads with the Vorpal Blade.
Gave 10000+ heads using the Powder of Growth
Decomposed the Decomposer as a Divisor of size at least 17.
Killed 8 hydras with an all-around attack
Drank 11+ Potions of Speed
Killed 11+ hydras with a single missile throw
Killed a Hydra with a giant blade
Got a blade of size 0 and attacked with it.
Defeat the first boss with a Human.
Killed a Two-Headed Giant with a machete
Killed the first Vulture Hydra honorably
Kill all Hydras on Level 5
Kill all Hydras on Level 1 without taking any wounds.
Kill all Hydras on Level 1