The achievement is unlocked when you kill the 100th enemy with an ability rather than a weapon. Each class has abilities that can be used. Some examples are Sorcery's storm bolt & ice barrage, Might's quake, and Finesse's Shadow Flare. The ability is mapped to a button for quick access during combat. It is activated by pressing
and the button to which the ability is mapped.
Abilities lack power at low levels but become quite lethal when enhanced. Nevertheless, the achievement can be unlocked using low power abilities by whittling down enemy HP with weapons and administering the coup de grace with the
+ability button attack. Even though the majority of the damage was done with the weapon, if the enemy succumbs as a result of a final ability strike, the kill will count towards the 100 tally required by the achievement. The below video illustrates weapons inflicting most of the damage but kills administered through ability attacks--the hundredth ability kill unlocks the achievement:
Alternating between weapon attacks and ability strikes is a good technique. The reckoning gauge recharges faster when alternating between weapons and abilities. Abilities tend to interrupt enemy attacks and/or slow them down. If one gets in the habit of fighting in such a manner, the achievement will be unlocked through normal game play and there will be no need to grind out the achievement.
Note that abilities have a cool down phase. So, you cannot spam the same ability over and over again. Switching between more than one ability and/or alternating weapon and ability attacks ensures the cooldown phase does not put you in a vulnerable position.