Hack and slash balance

Hello there. 

I'm working on a hack&slash and the gameplay/graphique parts are finished. 

I would like some advices/tips/rules to balance the level progression of my player/ennemies. I need clues to begin to think about it because its seems very difficult so any help would be appreciated. 

For your information i didn't set max level or the number of missions the player will face or anything else so any idea would be good. 

Thanks and sorry for my “english”