9th Dawn III

Unlike most RPGs, you don't have a random chance to get a critical hit when attacking. You'll need to use 5 Ability Coins to get the "stealth" ability. Put it in the hotbar, and use it to turn invisible. From there just attack an enemy to get this achievement. The levels of stealth seem to indicate enemies might be able to see you, so you may want to use a bow or other ranged weapon on a far away enemy to get this.