The Lava Lake!
"That's because we are using the wrong methods," William was watching these monsters all the time, "we can't harm them using the opposing elements of fire."
"Then what do you suggest, genius?" Sloth didn't like that William didn't directly say the solution.
"They are fearing fire, and so we need to use fire and lava elements in our attacks."
"You are crazy, right? Can't you see? They bathe in lava!"
"No, they are shielding themselves from it," William pointed to the glaring weakness of these monsters, "these shields are their strongest trait, and weakest soft spot as well. We need to use concentrated techniques of lava and fire, and you'll see how these shields are going to melt down."
"They won't die…"
"The lava underground will take care of them for us," William winked, "and if they have little intelligence, then they won't return back. So we can continue fighting them until they are down."