So.. Let's get started: I wanna make it clear in your mind that Diana is not a hard champ. All you have to do is just learn the next things:
-Your Q: Your Q is an important ability with your ult. When you reach lvl 6, keep in mind, that almost ALWAYS use your Q before you ult the enemy. I'll explain why at your R.
-Your W: W is another important thing for your ultimate. When you cast it, you will gain a little shield and some orbs. The shield is important when you're in early-mid game. If you are under their tower, use W for it's attack. The shild will absorb it, so don't worry about this.
-Your E: Your E is just a little slow down and moar attack speed.
-Your R: And here we are.. Your R. I wanna make it clear again: Nothing special, ez just like your Q, almost same cdr with your E, strong as your W.

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o: Combos are the most important things with Diana. Lemme show you some of these:

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Combo I: lvl 6 < lvl 18

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1. Q: Cuz you have no cdr on your ult if you cast on the enemy while he's got Moonlight Debuff.

2. R: A quick rush
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3. W: Shield and orbs while you're rushing
4.E: Don't let your victim escape
5.R: Rush again, but now there's cooldown
6.-And some basic attack

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Combo II: lvl 3 < lvl 5

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| E 1.
| W 2.
V Q 3.