Against diana, During team fights, Diana's kit offers a lot of burst damage thanks to her Ultimate R. Try to poke her down before she engages to make it really hard for her to engage and kill anybody.
 
Diana's engage is pretty scripted. Ward to the side when sieging an objective so she cannot surprise your team with an all-in.
 
Diana is an assassin and can quite easily one-shot any squishy target walking alone through the river. Do not enter the river unless you have it warded or know where Diana is. Against diana, During the team fights, Diana's kit offers a lot of bursting damage thanks to her Ultimate R. Try to bring her down before she engages to make it really difficult for her to engage and kill anyone. Diana's engagement is beautifully scripted. Ward on the side during a nap of a goal so she can't surprise your team with an all-in. Diana is a murderer and can quite easily a-shot any squishy target walking alone across the river. Do not enter the river unless you've kept it or know where Diana is.