Grimrock

Never bothered with those keys dropped by enemies, so I never ventured into the side rooms and saw any "lightning rods." Instead I went into a teleporter that lead me to the Tomb of Designers, where I was able to recover a weapon designed to kill the damn thing. I used it to pacify it so I could beat on it, then reused it when it started moving again. Eventually it died. (after some disassembling first, of course)

Things that worked well for me:
Lightning rod
Lightning bombs
Dismantler (Does lightning damage)

Ignore the critters, they respawn quickly, and you need to be running from the giant cube of squishy death ASAP. Shoot it with fire/ice/lightning/Ancient Weapon, and immediately hop into a nearby pit to rest and heal up; then teleport back up and rinse and repeat. It'll go down eventually.