OK, if you follow this step-by-step, the achievement is yours. It's story-related and cannot be missed, but if people are having trouble with it, my short guide should help you get the lightning power in no time!
Once the cutscene showing the BIG green fruit ends, first command the Majin to wait. Once that's done, DO NOT approach the first ladder that you see it and climb it. Ignore that ladder for right now. Instead, immediately head to your right, climb both the tree stump and the ladder, then get behind the dark eye and destroy it with your melee weapon!
From there, continue to your right, jump to the platform and to the ledge. You will see a box below, so just jump below and push the box to the other side. Once that's done, climb on the box and jump on the next ledge. Below, you will see two enemies. You won't have the Majin with you, so you'll have to deal with them on your own. Since they are shielded enemies, dodge their attacks by evading them, get behind them, then strike as much as possible. Rinse, wash, and repeat until both enemies have been slain. Do not worry about the dark eye or the big monster. They can't see you from their vantage point! From that fighting ground, jump on the next two ledges and get the nearby treasure chest, then make your way back, go across the dark railing, then slash the dark eye from behind.
After you killed him, jump to your left and you should see a shiny lever just waiting to be pulled. Instruct the Majin to come towards where you are, then you both get on the elevator and you pull the switch. You should be on the upper part of the area now. From there, head to your right and you should see a salamander. That's your cue to command the Majin to kneel before it so that you can jump to the ledge above you. Follow the walkway and hop over to the barrels. Below you should be an enemy and if it's dark, there should be a memory shard right next to you, so pick it up.
From this point, you can either take the exploding barrels and kill the enemy below with it or tackle him head on, it's your choice. I would recommend using the exploding barrels, since it doesn't take longer to kill than having to stab it multiple times before it's dead. Either way, it's your choice, but once he's dead, jump down and get behind the next dark eye you see and destroy it with your melee weapon. Then jump back and head towards where the Majin is waiting.
From there, if you walk to your left, you'll see an entrance teleport door that's currently blocked. To the left of that entrance is a ledge. Instruct the Majin to kneel near it so that you can may jump to that ledge. Grab the red, glowing fruit and head back down. Give it to the Majin to increase his strength.
Instruct the Majin to wait at his position. You remember the first ladder that you saw? Since you cleared out the entire area, with the exception of the big monster at the center, get back to ground level, and run to that ladder. Climb that ladder and immediately head to the upper-right of that part and immediately tap B rapidly when prompted. Hit it enough times and the bridge before you should extend to its fullest. Then turn around, command the Majin to take on the big beast, then you go ahead and fight yourself. Eventually, you will take down the giant beast and he will be vanquished for good!
Then go ahead and command the Majin to take the green fruit. A cutscene should ensue and once that's complete, the achievement "Lightning Majin" should unlock right then and there!
I hope my guide has aided you well! Have fun and good luck!