The Agitator Majinis' are the guys dressed in black and white with the megaphones. If done correctly, there will always be a cutscene to announce their arrival. There are only three found in the game, so if you miss one you’ll have to start all over again. Check out the video below which nicely lays out the locations and requirements to make each one appear. Note that the video describes a glitch where the trophy is obtained after only killing two of the Agitators. Unfortunately I was not so lucky and had to deal with all three for the trophy to pop.