Kingdoms of Amalur: Re-Reckoning

The achievement is unlocked when you kill the 500th enemy with an ability rather than a weapon. Before earning this achievement you would already have unlocked:

The only difference between this achievement and Shock and Awe is the number of enemies killed. You are required to kill 100 enemies in the earlier achievement and 500 enemies for And Then There Were None.

Each class has abilities that can be used for kills that count towards the achievement. 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.

Alternating between weapon attacks and ability strikes is a good technique. The reckoning gauge recharges faster when switching 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.

In the below video, the player character is at level 28--a hybrid sorcery/finesse destiny of Spellcloak. The destiny provides +30% elemental damage. Storm bolt and ice barrage enhanced by chain lightning and frostshackle respectively are even more lethal with the boost in elemental damage. They provide effective crowd control interrupting enemy attacks. The player character, though vastly outnumbered, sustains minimal damage. This is a very tedious achievement if you grind it out. I recommend unlocking the achievement through normal game play by incorporating abilities into your tactics and becoming more survivable and lethal in the process:
Note the achievement tracker in the video's lower right hand corner. You can use it to check your progress towards the achievement.

Here’s my old solution from the X360 version of the game, that should work just as good for this version:

If you're like me and played through the main questline as Finesse/Might you might need to grind this. Here's the quickest and easiest way I found to get this achievement:

1) Visit a Fateweaver and change your spec to Mage. Hopefully you've saved some Mage gear in your stash, though it's not a must.
2) Change difficulty to Easy (NOTE: don't do this until you've completed the game on Hard if you're working on that achievement).
3) Go to the very first town; Gohart.
4) Commit a crime and resist arrest.
5) Stand outside the Pub and lay waste to the never-ending spawning of guards.

The guards were spawning two at a time for me as soon as the first two were killed. I got this achievement in a matter of 10 minutes with this tactic, and I barely had any ability kills from playing through the game normally. You'll one-shot every guard, just use Frost and Lightning magic constantly.

In order for this achievement to pop, these must be active mapped abilities. Mines from the Finesse tree do count. I've found that the easiest way to do this without grinding is to finish off enemies with extremely low health with the shock ability. You can grind it easily with a Sorcerer if you prefer.