The Chimera's Curse: Bk. 4: The Companions Quartet by Julia Golding

It's a long hot summer. Temperatures are soaring and a strange creature has been spotted on Dartmoor. Connie Lionheart and her friends soon discover that the creature is a chimera. When it attacks Connie, leaving her gravely wounded, Col and Rat must rescue her from hospital: she needs treatment beyond the knowledge of mortals. But a weakened Connie is easy prey for the evil shapeshifter Kullervo. He believes that, finally, the time is ripe for him to seize her Universal power, gather his army of mythical beings, and wipe out humanity. The battle between him and Connie will be a fight to the death. And the prize will be the future of mankind.