I think the hardest boss for me was the miniboss
(http://theguardianlegend.com/clone/sprite-compendium_files/minibossredplant.gif) :skull: :skull: :skull:
also I remember I had problems with:
(http://theguardianlegend.com/clone/sprite-compendium_files/bossgreenclawbot.gif) or some of this type. It was time when I played the game as chiday story and I had no idea how to open corridor 4 because it was chinese language. After I finally beat this boss I thought it will help me but ohhh.. how dissapointing.. nothing happend... :(
After few months later when I got english language cartrige I had problem only with (http://theguardianlegend.com/clone/sprite-compendium_files/minibossredplant.gif) or some of this ones.
It was over 15 years ago so I hardly remember :)