The best way to defeat the energy leech is to keep the freeze ray on you from the previous level, & as soon as the fight scene with the leech begins it will slam its body on the sea bed, when it does zap the leech with the freeze ray and the leech's energy will be reduced down to the red, so you will only need 1 hit with the RPG to finish this beast off giving you an easy 35 gamerscore.