To decrease an acute stress-level of 8 or higher on a scale 1 to 10 where the fight-flight mode is controlling the body and mind, you have to focus on physiological techniques since at this level of stress we are outside of pedagogical reach.
The most important technique is slow breathing focusing on prolonged exhalations that are twice as long as the inhalations. Breathe with your stomach, in through your nose and out through your mouth in a relaxed manner. It should not feel frustrating or artificial. Do not worry about how to inhale since that will be an adjusted reflex. There are tons of breathing techniques, but I stick with simplicity because that will most likely be recalled and doable when seriously stressed. Breathing like this signals to the body and brain that the danger is gone and the body starts to get rid of the stress-hormones which slows the system.
Another technique is to apply ice-cold water in the face and neck, suck on ice cubes, rinsing your hands, palms and wrists with ice-cold water. This contracts the blood vessels, signals that the extra blood supply in the limbs is no longer needed and hence pushes more blood into your frontal lobes that are used when aiming to take control over yourself. This technique also works surprisingly well for patients with panic attacks since being active is often easier when the nervous system is aroused.
Another technique for breaking an overstimulated nervous system is muscle relaxation since it is difficult to get more winded up/having a panic attack with a relaxed body and slow breathing. Brisk walks burn energy and reduce stress hormones. Being in the nature is specifically stress-reducing.
Also, try distractions from your thoughts – like listening to or watching something that catches interest on a moderate level, which therefore do not add stress while already present stress dissolves.
Also, body contact – to be held or touched including holding the hands of someone you care for, even petting animals, have been shown to reduce stress, as well as a soothing and comforting voice.
See next session for how to reduce a heightened stress-baseline