Hello!

I'm a midskill player, I'm playing at 1900+ elo in EUW. This is the build I use for diana.

I used 21/0/9 and not 21/9/0 because I like the extra mana regen, it helps you a lot early game as you can spam your Q.

I use armor runes+regen quint because most of the time you'll be facing a range AP carry, this way you don't take too much damage from Autoattacks.

Deathfire+abyssal is enough to deal a lot of damage on ennemies ad and ap carry so I aim for GA next.

Then if the game lasts I take lichbane or rylai, however if you need more armor===>zhonya (it can be bought earlier vs champ like nautilus/karthus to nullify their ultimate) or more magic pen==>void staff.

Haunting guise is also very good early game, but I like to get abyssal ASAP for the magic resist. You can buy both, this way you deal true damage on ennemies with low magic resist.