Elemental Mercenaries were assassins commanded by the Rutans. It was rumoured that they were Rutans who had lost their form, only capable of shifting between elemental forms.
The Rutans sent an Elemental Mercenary in answer to Major Kreg's message demanding that they meet him in battle in Swansea. In the form of grass, it attacked Ianto Jones and then took on the form of fire to try to kill him and Kreg. It was unsuccessful and was defeated. (AUDIO: The Great Sontaran War)