Depends if you need to win a 1v1 duel or in bigger teamfights. If in Dominion it'll probably be more 1v1 or at most 2v2, so...
I'd probably try Nocturne against her. She needs to get close, and that's where you're deadliest too, your W absorbs one of her skills, your E will interrupt her combos, and your R has much better utility map-wise (really Noct is such a beast in Dominion, cutting off almost-taken points, I wonder why people don't play him more). Also you can build Odyn's Veil / Maw of Malmortius / Witt's End to counter her raw AP damage and round out your own offensive build.
Kassadin is worth a try too, you can Silence her, multi-flash away, and your passive heavily reduces magic damage, which is Diana's main form of damage.
I wonder if Leona and Diana don't happen to lock down each other. XD Depends on M Res to AP ratio maybe. Leona has enough CC to cut off Diana and let her allies focus her down. That'll be better on Summoner's Rift too, obviously.