There are a total of 9 enemy types that you must kill with the Maul (sledgehammer) in order to unlock this achievement. You can lower their health with other weapons, the killing blow just must be with the Maul.
Cultists:
Warrior - most common of the Cultists.
Herald - carries special weapon, such as Nano Rifle or Rocket Launcher.
Shiva - large robot-like enemy.
Aliens:
Creeper - small bug-like creature.
Alpha Creeper - larger version of the Creeper.
Ravager - majority of the enemies you’ll be fighting.
Wraith - glowing white alien that distorts your surroundings, disappears and teleports around.
Berserker - larger alien that runs towards you and explodes when near death. It’s important that you kill it before it explodes.
Behemoth - largest of the aliens, looks similar to the Berserker.
The order each enemy is encountered: Warrior, Herald, Shiva, Creeper, Ravager, Berserker, Alpha Creeper, Wraith, Behemoth. If you killed each as soon as they were encountered, the achievement should pop after you’ve killed the Behemoth.