In order to obtain this trophy you must kill a total of 5 parasites while the train is moving around the map.
The Parasites are the bloated yellow bugs that appear and spit the acid looking stuff at you.
The best method to accomplish this is to wait until they start appearing in mass quantities on round five, and then spend the 500 points to take the tram. Whilst the tram is moving between locations start shooting as many of the parasites as you can.
Once you get 5 the trophy is yours.
Here is a video by PowerPyx showing you how to accomplish this