The game misleads you about this achievement. When approaching the locked shed where the first Volcanologist is hiding you get a warning about being under-equipped (not having an electrical weapon) even if you have a Golden weapon in your hand. Which is precisely what you need to complete this achievement. 4 headshots with the Golden triple boomerang should do it (in Normal or about as many with any Golden weapon). You know you're damaging the Volcanologist's helmet if you hear glass cracking each time your attack hits.