From Halopedia, the Halo wiki
Stunning! is an Achievement in Halo 3: ODST. It is awarded for disabling any Vehicle with an over-charged Plasma Pistol shot and killing the driver of the vehicle. It is worth 5 Gamerscore. It is easily obtainable on the missions Kizingo Boulevard and Uplift Reserve and as this is not game mode specific it can be obtained in campaign. It is recommended to turn the difficulty down to Easy or Normal to earn this achievement with ease. It can also be obtained if a Brute throws a Power Drain near a Vehicle and you melee the driver.
It is represented by a gold 16-pointed star, with a green 16-pointed star within it, with a red circle, with a stop sign in it, in the middle.