This is actually pretty simple to do.
First, make an event with the NPC dialouge. Then, after the dialogue (in the same event) add the event command Battle Processing. You'll see a box that let's you choose which enemy troop the battle will be with, and whether the player can escape or not.
To add more feeling to it, you can use the Change Battle BGM command before the Battle Processing one to set some special boss fight music, and then after the Battle Processing command, use the same BGM one to change it back to normal ready for normal battles.
I hope that helps, but if I didn't explain it well, let me know.