Finally I've beaten the game. 5/5 because in the end it has enjoyed me and for the kindness of developers answering to all players and working on fixing bugs each day.
6th level for it's impossible to accomplish, due to those simple reasons: most of the times boss won't appear and health+dxp keeps flashing (after 2nd protection spheres upgrade...are they useful in some ways, however?). those with the fact that 6th boss isn't so easy made me stop playing this game until further notice.