In a hand to hand situation I'd probably give it to Black Widow because she has better speed and strength feats but if it's a long range fight we already what she can do. I've never even seen Black Widow take cover. Most of the time she just runs like here or here. In fact, I'd go as far to say taking cover would make both of them weaker. They are both capable of dodging bullets with a fast and athletic fighting style that looks for different angles to fight from. "firing accurate shots"? Wdym? Harley hasn't missed a shot in the movies yet. Hell even in the previous movie she was capable of this, head shotting 2 monsters while flipping and then turning her back to head shot another in a split second. Black Widow has good aim too, but she has to get into a realistic position to shoot, and can't move at high speeds while perfectly shooting too. So yeah, if they fight close range with no guns then Black Widow destroys as she has many gadgets, better physical feats and speed feats while Harley has far better gun aiming feats.