Increase the drop rate for the Underdark Arena challenges. They give a LOT less ingredients than the other challenges.
Or have an option to exchange ingredients, like the Cannith Challenges. Maybe any ingredient at 30:10, or a series of specific trades at 20:10.
The random weapons you get from the ingredients could be improved too: Add Paragon or Epic weapon, and give a short list to choose from (like an end rewards list).