I've gotten a couple Insane combos, I'm using the Dark Matter power and just X X X Y combo into a Phase Shift into another Phase Shift, that gives it to me if there are enough enemies. The VERY 1st time I got it though I used the RT B power on the way to the roof when you get attacked by all the mutant "Colossus" enemies all straight in front of you in a narrow passageway. (Near the end of the game) I hit RT B and took them all out with that and the cheevo popped.
100+ Retail Games_100%
100,000 GS (9/12/09)
200,000 GS (2/2/2014) (I blame MMOs)