ok, just took the quest "gym membership" and it says "try to find all (10) emerald pieces on ANY of the alien bosses"

i have gone thru Alien Oasis part 11, vicious visitors, four times and have not gotten a single emerald piece... fought and killed all the bosses

am i in the wrong place? what bosses do i need to kill to get the emeralds?



thanks every one in advance