I will answer the question for what it is......

okay bosses that can be hard your first go around.......

the gi cave bosses.
Demons gate
black dragon
prod cloud

Theres a couple more......but depending on your level and how far you like to wonder through out the game. Such as when i faught prod cloud I already had kotr...One hit death.....
Like none of these bosses are really to hard.....but i know so great stratigies for the game...and those were a few that i was asked about several times