"Light first bonfire with fire damage" is an objective in the side quest Lost Souls in Tiny Tina's Assault on Dragon Keep DLC
Walkthrough
Head to the East then North from the Crestfallen Player. This will lead you to the first of 3 Bonfires that you need to light. Approach and fire on it with a Fire-Type weapon to ignite it and trigger the conflict. You will encounter Conjurors and likely encounter Knights and Archers as well.
