You have to not have fully regenerated your HP and MP by this point for the exploit to work.
When the short QTE for the follow-up strike occurs, execute it. Execute a companion technique while you are regenerating your HP and MP (Prompto's Stinger works well). Then, find a spot to teleport and regenerate your HP and MP - it has to be one where Noctis basically hangs there with his sword. Enter a battle and take a substantial amount of damage.
