I think you have the idea going here with Madred's Razor and Last Whisper to help take out those tanks, but you seem to forget how incredible MF's autoattack is. I would gear her more towards AD instead of AP, because in the end, no matter how you max her AP, her auto is going to be a more reliable method to kill and take out towers. Damage off your abilities shouldn't be a prime focus, because even without any AP added, they still allow you to harass, slow and slow their healing.

On another note, her Ult can be a massive weapon to controlling a lane. Yes, Champions can side step it, you must first drop your E on them, and catch them about mid distance of the cone to minimize their ability to get away. If that doesn't work, use it to catch them and minions to push them back and make them think twice. ;)