Stunning is an achievement in Metro 2033, awarded for getting 25 kills with the alternate fire of the Volt Driver. Kills towards this achievement can be easily accomplished, as the alternate fire of the Volt Driver is strong and can kill most kinds on enemies quickly. The kills can be accomplished on both mutants and human enemies. This achievement requires the Ranger Pack DLC, as this is the only way to obtain the Volt Driver. The kills must be in one play session, and are not cumulative. Be aware that using the alternate fire of the Volt Driver rapidly reduces its charge, so at full charge you will get five attacks with the alternate fire. This differs from the Metro 2033 Redux achievement/trophy, Stunning, awarded for knocking out 30 human enemies.
The alternate fire of the Volt Driver is a basic thrust, but causes high damage. This is primarily because the tips of the weapon are conductive, causing shock damage to the target. These attacks will usually kill an enemy in one or two hits, even on lower difficulties.