I love Bruce, but I think the only way he would ever beat Diana is if he had a good day's worth of warning to enact something. After that he would go into a siege mentality (use the City he knows so well as the place of battle), but there's no guarantee that would save him either. Unlike Clark, Diana has no outstanding weaknesses.

Bruce works better not as the Trinity's best fighter but as an agent. He is the detective, the spy, the saboteur. He does the work that Clark finds so distasteful and that Diana would not do unless pushed to. Bruce in a meta-human fight should always be the wild card - you don't quite know where he'll come from and who he'll target. While he can hold his own against young, inexperienced meta-powers, it wouldn't be smart for him to be fighting in the thick of it. Careful selection, quick thinking and clear and controlled action should be his watchwords.