Right! Well, nothing flashy or anything, just like have a window that displays [variable number/Amount you started with] Like, say the boss has 500 health, it'd look like [500/500] in the window, then you attack him once, and it becomes [499/500] and so on. Pretty simple.
So to elaborate, I made a working fighting system with events, for boss battles I wanted the enemy's health or variable to display just so the player can know when they're close to winning. I hope that explains a little more in depth. :3