Don't just stack Rabadon's because of it's 140 Ablilty power and the Unique passive, Unique means it doesn't stack so just dont stack Rabadon's Deathcap for her passive, it's an lame way to play her!
You can get the dmg up on 1000 dmg, with 6 Rabadon's Deathcap i tried it against bots and it was not fun because i could go 1 vs 5 and get out only lost half of my hp, so it would be a lame way to get her passive, to do high dmg.

ABILITY POWER
In my Ability powered, build/guide, the passive's dmg will be 750 dmg, so that's quiet alot, ecspecially when you have 50% Attack speed, so my guide/build will hit twice as fast if the attack speed wasn't there, and twice as fast than most of the other guides/builds in here.

HYBRID
I dont know how much the passive hits but i it's 60% of you'r Ability power, so i will guess it around 400 dmg. And you have to know what you are doing because you dont hit that hard with you'r abilitys, so have to use them wisely, but you'll hit alot fast because you Guinsoo's Rageblade and Nashor's tooth and Trinity force.

So i hope you liked my guide/build. so plz give a feedback if you want to know more about Diana, and i will answer as many questions as i can.