Killing metalheads make them drop a yellow orb. This will only happen once per monster, so if it respawns it will not drop it again. If you pick up as many as you can during your normal playthrough you should be able to get this before the end. But if not, you can get it after you finish the game. So just play the game as normal, and if you haven't got it by the time you finish the game, the metal heads will have respawned with the orbs and you can easily get the last ones by going to some of the areas where you fought them before.