The "Hephaestus Room" is located in the level "St. Francis Frolly". When you get into the Hephaestus Room you will see a dark ball zapping floor tiles as the elevate and descend into the floor. The dark ball only zaps the floor tiles that are elevated. To get past this and open the gate you have to press the four floor tiles that are smaller than the rest. Just make sure you're not standing on the elevated tiles about a second after they elevate and you won't get zapped. Make sure to save as you enter the room so if you mess up you will just have to reload the save to try again. You will receive the achievement when you exit the room with the key of hephaestus.