Need Zandatsu help in VR 19

I just finished VR 18 finally after two straight days of nothing but trying to complete it over and over and over, and now VR 19 is giving me problems. Not because I can't beat all of the enemies but because I have a hard time knowing how to trigger Zandatsu. So far I've really just been guessing when I need to go into blade mode and trigger it. I need someone to explain in detail how I know when I can trigger Zandatsu. Sometimes I can trigger it by parrying but I need to know in general how to know when I go into blade mode that I will trigger Zandatsu 100% of the time instead of just beating an enemy up a little bit and then just hoping I get lucky and the little red box shows up when I go into blade mode. I saw in another forum you know you can zandatsu when you see the ninja run queue but I"m not even sure when the ninja run queue is..I want that Yellow suit!! Thanks.