The earliest level to get this is the final chapter (Port) of mission 2, The Passing. You spawn almost immediately next to the bridge from the final chapter of L4D1, and three of the four original survivors will kill infected from above with guns, snipers and machine guns. Tanks are very common in this level (especially when time progresses and waves intensify), just lure one over to the section near the generator where the original survivors can shoot it.
We got this by accident in the middle of a wave where the common infected were crowding the Tank after I threw a bile bottle on it. This prevented it from killing us as well.