So, if I go AP Rengar

no ap shield

It's supposed to be based on the AD/AP ratio of the damage you dealt durning the game compared to her own resistances.

For example if she has tons of MR that despite the fact you're AP, she could still end up with a physical shield, because she'd negate so much of your AP damage, that the AD part would end up dealing more damage.

But for the most part if you are building AP - and you actually scale with AP - then she'd get a magic shield.