You have to know that you can always track back in this video game's world - also from the last Save Station that is. Now for the Solution regarding this Achievement and more:
Seek a place where three or more Darkness Enemies gather. Send in the Majin with a Command to unleash Lightning. The Darkness Enemies will be suffering electric shocks. Then head to one or more of those elemental infused Darkness Enemies, and cause damage on them for only a few times - it would be great to damage more than one of them. Then you shall bress B in order to perform a Combination Attack (Tepeu & the Majin together) which is then also based on an Elemental Power. If you have even damaged another Darkness Enemy just before, you shall do this performance once more - if not, it is required to damage an Darkness Enemy, before you can do. If this has not yet been a Finish Attack that is also based on an Elemental Power, surely your next attempt will be, providing you were fast enough to get the Combination Bar full, and then displayed in its full state onscreen. In the case Darkness Enemies get back in battle action, you shall let the Majin unleash Lightning on the enemy again - swiftly build up the Combination Bar like this is most efficiently done the way I have instructed you to do actions. If the Majin's Elemental Power is short, you are unable to use it - damage Darkness Enemies, to reveal Darkness for the Majin to absorb, to fill the Elemental Powers Bar - this also helps to fill up the Combination Bar. The Combination Bar depletes after times solely - it can be taken from one battle, to the next if you are fast.
When a Finish Attack is performed, all enemies in the widened area of effect are defeated - only ten enemies have to be defeated that way, no matter if separately. While you are building up the Combination Bar in order to unleash a Finish Attack that is even based on an Elemental Power rather quickly, it can be that Darkness Enemies with Shield and Sword will respawn at certain places, if you had defeated those during the fast actions. This means, that you never have to worry much about quantity of enemies. Just simply perform systematic actions to succeed fast and efficient!
The place to go - the best place - Arkela Castle, Main Tower (when backtracking): When passing by the Save Station when this one is closely placed to a wall, head out from the opening to the right, and see the six Darkness Enemies in the room to the left from the area you have just entered. That is the ultimate spot to defeat Darkness Enemies in order to do whatever you may miss, or just have missed to progress on Achievements' requirements regarding battling - besides Trap-related performances. When the battle is over and you still have to do more of such, you shall go back to that Save Station and select Change Outfit, after one second you press B. Go back to the six enemies. Do whatever you have to. After one hour, it is likely that you have finished with everything about battling - except Trap related stuff. I got one more battle input for you: The Darkness Enemy with the long arms is defenseless to Attack - it helps you when building up Elemental Powers Bar.
Hint - get three similar Achievements like this one: Do the same with the others of the Majin's Elemental Powers.