Sort of like "The Sampler", but a little more strict and difficult. You need to kill 4 enemies in under 20 seconds. You also need to kill each one with the item listed in the trophy description. Since the vehicle is not something you have available to you all the time, you need to find a point where you have the truck in order to be able to get this trophy. The video guide below will explain how to do it in detail, but the requirements are quite simple anyway. There are other places you might be able to manage this, but you need to make sure you do it quick. The 20 second time limit is what makes this tricky.
Defeat enemies with the vehicle, long-gun, melee, and grenade, in that order, in 20 seconds