Bring someone who has a fire spell to make quick work of the Ice Flan. The Lamia and Icedrakes are the biggest concern, so choose one of those three to focus on, kill it, and move on to the next. There is also a Bomb, but it isn't too hard to kill.
Once you're done with the mission, Marche sees diamonds falling from the sky. He tries to grab one, but it disappears.