I think they would both have equal footing at first, but I'm sure if work and fight together long enough beforehand, Leliana is the type of figure out her opponents weaknesses and exploit them at the right moment. She probably won't fall for Zevran's attempts at being charming either during the fight and maybe when defeated. On the other hand, Zevran does have the Crow training under his belt and I'm sure he can hold his own during a fight.
Sorry Zevran, it's close but if given the time, Leliana will ruin your day!!
This is a close call, but I think Leliana wins out. I mean, Zevran was a member of the most feared group of assassins in Thedas, but Leliana became the Left Hand of the Divine, right? That has to count for something -- they wouldn't pick any old assassin for that. So, while I'm not sure who would come out on top in a straight-up fight, I'd put my money on Leliana's wit and experience over Zevran's.
Maaaan this is a tough one. I think Leliana would have the upper hand, though. I think she has a level of cunning that Zevran doesn't quite match, and after spending time with him and fighting alongside him, she'll know his moves and would be one step ahead of him.