Build your shield and get "Victorious Tortoise" perk and "Stone Cold Stronghold" in the modified slot (fourth). Press L3 to toggle the script. Place yourself anywhere as Victorioius Tortoise will protect you from every side. Once the shield breaks the glitch wll start and you will be invincible. Don't pause the game as it will stop the glitch. If this happens just go and buy another shieldand toggle the script again.