Build and Runes
Dodging her Crescent Strike will lower her damage output significantly after level 6.
Her Shield lasts for only a short while, so wait until it wears out and harass her after.
By using a knock-up (e.g. Blitzcrank’s Power Fist) and/or displacement (Tristana’s Buster Shot) while she is using her ultimate on a marked target will deny her a reset on her ultimate.
Diana works well with a team that can follow up with her (e.g. initiate.) If your enemy team has a team like Orianna, Diana, Leona, Xin Zhao & Jarvan IV for example. Hard Crowd Control (AOE stuns) will easily shut down Diana, which can shut down her.