Mind the Gap
Halo 3: 4 vehicles, 1 gap on Legendary using Cowbell, Catch, Thunderstorm, Mythic and Tough Luck.0
Mission: Tsavo Highway - Mind the Gap - This achievement requires you to get 4 vehicles, ANY 4 vehicles, across the afore-mentioned gap in the bridge.... on Legendary.... with Cowbell, Catch, Thunderstorm, Mythic, and Tough Luck skulls active. It can be done solo or in co-op, but obviously co-op will make it much easier as you have friends to drive the vehicles through the level with you and you don't have to restart the last checkpoint if you die. In the absence of friends, you could use a second controller as a spawn point to save some hassle. There are at least 4 Warthogs, one troop transport, and several Brute Choppers in the first portion of the level before the gap in the bridge. I prefer to bring 3 hogs and the troop transport as it's easier to get these through the concrete block gap than to get choppers through it. The video below shows this achievement in action, starting at the beginning of the bridge.