The challenge is doable.. I've done it on my guardian. I need to try it on my hunter, though. You just have to be REALLY careful to kill the remaining mobs quickly (the ones that start the books burning) and then be careful about where you walk to put out the books. If you go outside of the tiles that the books are on you can aggro the boss once he's up.
You have to choose your path wisely. You have very little time to extinguish the books.
Chromite (Hunter) - Grumbletocks (Guardian) on Landroval, Appendage (Hunter) on Brandywine