Now in the arid badlands of Shatterscarp, the Envoy has reached the beleaguered town of Thirdborn and re-established contact with Yatzli to find A Path to the ...

Obviously, you will use them in Elemental Tower as well. Also, if you don't have a solid third team, you can use them for that as well. Other than that, they have no real use. C-tier heroes are food.