Instructions
Explore the cave with the Crystal Crab until you have two blue crystals. These are needed to set a trap for the crab.
Walk to the barriers found in the north side of the cave and press "E" in front of each one. You will insert the crystals you found into the barrier and create a lightning trap. The crab will have to be lured into the barrier for the trap to work.
Walk to the spot where the Crystal Crab appears and you'll notice a pot filled with oil. Stand in front of it and press "E" to tip the pot and spill the oil on the ground.
Stand back then right click the oil on the ground to set it on fire. The Crystal Crab will emerge and chase you. Run down the hallway and past your lightning trap.
Wait for the Crystal Crab to get within range of the lightning trap then press "E" in front of the lever next to the right crystal to activate it. The crab will be fried by the lightning that shoots between the two crystals in the hallway.