I beat the game long time ago and If I remenber you have to level up the main character. It going to spoils but, firast floor you need at least LV4 but go to next level you need at least LV6.
For the 2nd floor you need at leat LV6. then go to the next floor you need LV11 by the way you need LV8 to beat the 2nd floor boss.
On the 3rd floor you need at least LV11 to defeat the enemy and LV13 for the boss .
In the 4th floor you need at least LV16 and beat the boss you need LV17 hints use the thunder slash and potion and you ca beat the boss. Anyway, you need more level for the raidy. I can post more but, some people hate about it so if you need more information PM me.