Let's start with the concept shall we  ?

First of all, we want to be able to play, so we need a character, a normal enemy and a boss. 
At first, there is no health mecanics. Only the player can make an attack that kills the enemy so the dungeons mecanics can be triggered. 
Here, we need a certain amount of enemy killed on the first zone AND the first zone boss to be dead so that the dungeons second zone can be open. 
Here's a little video showing that. 

So what do we need to do in the dungeon to finish it  ?

By default, each dungeons will have 3 zones. Each zones have a certain amount of enemies that needs to be killed and it's boss to allow the player to get to the next zone. 
Once the third zone is cleared and its boss it dead, the dungeons is over. 
There is also a timer that the player will need to beat. 
Beating the timer allows you to get loot to grind your equipment. 

How to get to a boss zone ? 

So as I said, each zone has its boss, but the boss is not where the other enemies of that zone are. 
Each zone has a dark portal that teleports you directly to the boss. 
You can do the boss first and the enemies after, there is no restriction on that part. 
If you are having trouble fighting a boss, you can take the portal to leave it's room, but if you do, the boss gets back his health fully.