Devastate is pretty much the name for overkill on this game and will easily be awarded during your 12+ hours in Lemuria. But to explain overkill, just like your party them enemy have health and when it drops low they will begin to bend over as if they are going to faint. This is how you can tell that they are about to die so when you see this, unleash your best attacks on your foes. I personally got this achievement after I gained the ability to fly :)