took a few tries but I did it on Snowbound during a BR slayer match. Basically I took the Mongoose and drove it around the outer edge of the map within the trenched areas as to avoid being too exposed and fortunately I was able to catch a guy running at the right time. And I believe he had full shields too as he appeared to not be in combat. Here's a clip you can download to Halo 3 to see how I did it.
http://www.bungie.net/Online/Halo3UserContentDetails.aspx?h3...