If I have to go against diana I usually use viktor. You can poke her down for days with your superior ranged lazer and just make sure to juke the crescent strikes. At level six you can let her land one, which is where you lay down a gravity field on yourself, flash out and then ult/q/e/ignite her and you can usually pick up the first kill of the lane.

Take AD Runes and masteries on Malzahar. Don't take AD items later on though, late game AD Malz is lacking serious power. Take flat AD quints, flat AD marks, Armor yellows, and MR blues. Voidling will make Diana cry early, ganks on Malz are impossible pre-six and they usually end with a double kill for Malz. After six and his ROA, he's invincible. She dives, he combos, she dies.