Diana is one of the highest burst champions in the game. She is probably one of the worst ap mid champions before she becomes level 6. However one cs away from level 6 you should get ready to engage your opponent. If you had been chipping your opponent until you become level 6 you can easily out damage your opponent. If you are successful you will be able to easily win the game unless you make countless many mistakes. Since Diana is really level dependant if you fall back a little it will be very hard for you to win the game.