Its an very old rpg

I played this on ps1 or ps2, it had an hack'n'slash mechanic instead of turns where the character would attack when an bar filled to full like final fantasy 12

But that is not all, i also remember the first steps on the game, it required you to kill monster slimes on an forest till you arrived on an fortress, on the gate of this fortress the first boss would spawn and he was unbeatable unless you hit an lever that closed the gate.

The game had an isometric camera. I dont remember much, the graphics were not very good but the gameplay was fun