Record of Agarest War Zero

To get an overkill, you need to completely fill the blue bar in the enemies health gauge. The blue bar begins to fill after the enemies green health bar is depleted, and is increased by doing damage to the enemy.

The easiest way to do it is find a weaker enemy and use a linked attack to hit it multiple times. Make sure all your characters are in a glowing box, and use the LT/RT to cycle through and give everyone an attack. If you're worried, save up your AP for a turn so everyone can get 2 attacks. Also, attacking an enemy near death will make this easier as well.

I achieved this in the very first fight once you are allowed to move around the map after recruiting Mimel.