A 25-year-old female is found unconscious at a music festival. Her pupils are dilated to 8 mm bilaterally and are reactive to light. She is tachycardic (145 bpm), hyperthermic (40.5°C), agitated when stimulated, and has marked clonus and hyperreflexia. Her friends say she took MDMA and has been on sertraline for depression. What is the most important immediate treatment alongside active cooling?