Dungeons & Dragons: Chronicles of Mystara

To get this quickly play, Shadow over Mystara. In this game the Elf & Magic User start with a spell called Lightning Bolt.
*The Elf in Tower of Doom does not start with a lightning spell.

Cast the lightning bolt, preferably when multiple enemies are on screen. The lightning only has to hit an enemy to count, it doesn't have to kill it. If playing as the Magic User, you have to press the jump button once while your inventory is up. This switches the wheel to the next page where your lightning bolt spell is.

It can also be earned by using rings, rods & wands of lightning, but you have to collect them during your journey.

I've also discovered the Magic User's Conjure Elemental spell counts towards the achievement/challenge. Select the black clothed Magic User by pressing on the character select screen. When you face the right direction he summons a Water Efreet, which whips up a rain storm. I guess the enemies armor is charged and reacts in contact with water.