Random and a total pain. I went for this while also going for Mac & Blue's achievement
on the Final Battle stage, because one tap of X will shoot someone off the screen. This will get you crazy amounts of money as well, helping you get the three "buy everything" achievements. I set the timer to 10 minutes and just ran back and fourth knocking off dexter and chowder (thus going for his achievement as well) until the timer ran out. It ended up taking me about 20 minutes to get all three of these ridiculous achievements.
As a side note, Mandark's assist seems to be the rarest, as it was the very last achievement I had in the game. Deedee appeared twice and Chowder's girlfriend appeared two or three times as well, while Mandark only appeared once.
Good luck, and stay sane.