After you beat the low-level boss mission, you will eventually open up a medium-level boss fight mission. After you beat that you will eventually open a high-level boss mission. The bosses have similar names across difficulty levels. You can see this by scrolling through the list of the Keeper's missions/achievements.
Once you can build a party that is sufficiently strong enough to fight a boss, you should attempt the dungeon. Recall that, for example, level 3+ heroes cannot participate in a level 1 mission. This means that if you wait too long to fight a boss, your party members may become too strong and you will have to train up new ones.
One tip: For the low- and mid-level boss fights (I haven't reached the high-level fights yet), the Hag is MUCH more difficult than the Swine or Necromancer of the same level.