One of the quickest ways to get this achievement is with the Warrior Class and the skill, Cleave.
Cleave will deal do damage from the skill + the damage of your weapons.
On the second level of the game, The Great Forest, you will cross a bridge. When I crossed the bridge I encountered Skeletons with the same amount of health from the first level, 16~20 HP. Your skill does a base Damage of 8-13, your Min Damage requirement for weapons will be 7 damage, the higher the better
If you are unable to kill all the skeletons in one hit, there should be more in the cave to your right.
I would recommend saving shorty beforehand, so you can reload in case you can’t kill all the skeletons in one hit.
It’ also possible to do this is the first level, Lake of Anna, if you unlocked Cleave early and have weapons that deal 7+ Damage.
Edit: Made some changes on damage requirements.