Already sorry for the bad english, I'm french.

So... Diana is a magical assassin. She must engage in battle with her Z-R, protect herself with w, And repeat until the opponent die.

In early game, the A quickly consumes mana, that's why I recommend the Calice of Harmony, who will become the unholy grail later. But if you have enough gold to buy the Large Rod, do it.

Passive ability at Diana is especially useful in farming, during the early, because spells inflict enough damage to scare looser, and protect against the strongest time of the CD, It is more useful to focus on power, in order to kill the minion at one hit. In PvP, it finds use when enemies can withstand a little more spells, that for what the Nashor's Tooth is at the end. But if the need arises, you can take it earlier.

Diana excels in the Art to go behind the enemis lines for kill the carry. If the latter is not catchable, the Z and the W inflicts of not insignificant damage in area, and if your tank doing a good job, you can go into the melee with your Z-R-W, place a passive, do it again, kill the fugitive with E, or your R if it's too late, and win the figth.