How was it PIS? The guy planned the whole thing out and executed it like the hardcore villain that he is.
He used all their obvious weaknesses and the only real PIS, is that Deathstroke lost.
To quote what I said about Deathstroke's fight being PIS, vs. Prometheus' fight being considered 'real', in another thread:
From what I've seen here on CBUB, and what I know of the two characters, the like/dislike comes from the fact that a lot of people don't understand Deathstroke's power(s), while Prometheus is a late 90's creation, and following the 90's trend is an 'easy to access badass'.
If you don't understand Deathstroke's power(s), then he's essentially just a normal human with slight enhancements, and from that perspective(however wrong it may be) he's nothing special, he definitely shouldn't stand a chance against any super-powered group of JLA members.
While with Prometheus, everything about his 'power(s)' is explained and easily understood, with equipment that directly targets specific weaknesses of some of the more powerful JLA members. It's easy to see how he could defeat a group of super-powered JLA members.
Hope that explains it for you.