Ruka seems to be the one person (other than Mikage) who approached the duels in a semi-scientific manner.  He knew off the bat that his duel with Shiori by his side probably wouldn't work and noticed that Anthy probably had a lot to do with Utena winning even if he didn't quite know how.  It seems his one duel was basically to test out a theory (and to humiliate Shiori).  He seemed to know that it had nothing to do with skill. 

It makes me wonder how much Ruka knew, whether Akio told him anything or he puzzled it out for himself from what he heard about the duels.