Let me tell you that for the same level of the creatures as your characters lvl, it's expected for them to have more than one fight "per day", so *not as challenging as a Boss battle* is understandable. 

Remember, your characters are there not because they are common people, but because they are great, **let them feel their greatness** and further involve in the game.

It also happens that dices are trickier than you might expect. So don't calculate on above the average rollings. 

Now to answer your question: <h3>Is it Normal? </h3>

<h2>That isn't intended to be your day to day basis</h2> However you can make them as challenging or un-challenging as you want/need.