Pretty easy to execute glitch I discovered by accident. 1. Enter a battle 2. Take damage(a substantial amount) 3. Look around to find a spot where you can teleport to and regenerate, has to be one where Noctis basically hangs there with his sword 4. Execute a companion technique while you're regenerating (Prompto's Stinger works well) 5. When the little QTE for the followup strike happens you have to execute that as well. You have to not have fully regenerated by this time. HP and MP regeneration now keeps happening and only ends if you end the game or load a save. Note that you are not invincible though, enemies with huge damage can still rush you down and the regeneration doesn't work if you're in 'Danger'.