In The Monsters Den, you can choose Boss Clash, and beat different types of monsters.
However, the missions that relate to these battles, have started to "complete themselves", everytime I log in. Currently my mission is "Ancient Yeti Training", and I've completed missions relating to Boar King and Abominations. I've never beaten any of these monsters, I've only gotten to Soul Mage.
It's fun being granted the rewards for missions I haven't completed, but it would be more fun to complete the missions myself.
This is a kind of "bug" helpfull for the players,because you are granted even if you don't have the level to defeat the right target. Nevertheless this point has been sent to the programmers in order to fix it. I can't say that it has been fixed, but they try.