You need two people, one gets in a mongoose the other person gets in a wraith.

You need to go to the base with the beach, the person in the mongoose has to go into the water go till you cant go any more (end of map), but keep on driving.

The other person in the wraith go behind the person in the mongoose and use the burst and hit the guy on the mongoose.

The guy on the mongoose has to jump after he hits you (have to jump on time or it won't work), then you will be under water (out of the map), but if you go too far you will die.