There are no 'hard' bosses in most rpg games... a simple way of beating most of them is just to train yourself to the maximum level and they can all be taken down easily...
As to Soul Blazer... i know nothing about Jap and i had the Jap version of it (it's called Soul Blader there)... i had to play through the whole game without any game guides, only the instruction manual . It took me A LONG TIME to beat the game but i'm pretty sure i beat(and found) the first boss on my first time playing it.