First of all, you must have the gametype as cops and robbers.

You need 3 people, 1 to be the cop and 2 to be robbers. You can forge your own map, just make sure that there is a chopper and a mongoose.

Have the cop drive the mongoose and have the robber to ride in it (Change the settings if necessary) and have the other robber to be in the chopper.

Have the driver of the mongoose (cop/zombie) and the chopper to do a head on and a couple of seconds before they hit, have the robber boost in the chopper.

The result should be that the chopper's boost should kill the robber riding and the mongoose will be blown up with the cop driving it!

Tip: the mongoose will roll, it might take a couple of tries to make sure it doesn't roll, but it is a fairly easy cheat.