Stunning!
From Halopedia, the Halo wiki
Stunning!
Stunning! is an Achievement in Halo 3: ODST. It is awarded for disabling any Vehicles with an over-charged Plasma Pistol shot and killing the driver of the vehicle. It is worth 5 Gamerpoints. [1] It is easily obtainable in the missions Kizingo Boulevard and Uplift Reserve. It is recommended to turn the difficulty down to Easy or Normal to earn this achievement with ease.
It is represented by a gold 16-pointed star, with a green 16-pointed star within it, with a red circle, with a minus symbol in it, in the middle.
Sources
http://www.youtube.com/watch?v=OaPInHwc_mQ