Okay, in Detail, allow me to explain the best I can..
#1--> You start off in a hallway and at the end is a door on your Right. Go through it, and you'll come to a catwalk above ground-level, & bowl you can go down. On the lower ground is an Red Barrel of Doom, and a guy guarding the Gate Door..
Kill every Majini in this area, and throw a taunt or 2 in there to lure any more hidden Majini.. Explore every area of this outside-room before you go through the Locked Gate Door, and kill every Majini possible.. After you spend a few minutes doing this; unlock the Gate Door, and run to your right.. You'll come to a "Partner-Required Ladder".. DON'T go up the ladder yet.. Instead, turn around again and explore the previous area with the Red Barrel of Doom for anymore Majini.. Kill ever Majini in both these areas then use the Partner-Ladder.. OCCASIONALLY, a cutscene will appear once Jill goes up the ladder showing one Agitator Majini (Majini with Megaphone in hand). Run to the "Red Barrel of Doom Area", and find him..
#2--> At the Third-4'th part, (The outside ramp with TWO Turrets RIGHT NEXT TO EACHOTHER )if you look across the gap, you'll see 2-3 Majini with Crossbows or Molotov Cocktails shooting at you.. Kill them, and wait for more to appear.. STAY beside the two Turrets, and keep killing Majini across the gap and eventually, a Agitator Majini will appear with a group of 4-5 other Majini..
#3--> At the last part of the game where you have your Hold-Out on the rooftop with Majini coming costantly, and a 7-8 minute Survival-Timer, kill as many Majini as you can.. The first 5 minutes will go out, then a new 2-3 minute Timer will appear after Doug messages you saying he ran into Flying BOW's. At about 30 Seconds left to survive, an Agitator Majini will come up the elevator with 2 Big-Man Majini's.. Kill this Agitator Majini, and a Cephelo WILL pop out unless you use Suplex or Neck Breaker. Quickly kill this Cephelo before the timer runs out! Use a "Flash Grenade" for instant-kill.
Also, do they not appear in Amateur mode? My first playthrough of DE was in Veteran mode. That is when I saw the cutscene with the first agitator. I set it to Amateur, though, for the sake of getting this trophy quickly, and I just cannot get the first agitator to spawn, even following your instructions. I know you say it happens occasionally, but it seems very silly to offer players only a small chance at a particular trophy in any given playthrough. So, I'm thinking he simply might not appear in Amateur. Do you know?