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