2 items designed majorly around making tank items not so tanky, and you are expecting to not take a lot of damage?
You only have 175 armor, that's hardly anything in a tanky perpsective. You're not a tank, you're a bruiser.
Also for mfs ult to do that much, it has to be full channeled and she has to be level 11 with at least 280 AD.
Her level 2 ult with 280 ad will do 2,940 damage before resistances, after LDR you'll have around 145 armor and after black cleaver you'll have 102 armor.
That's still slightly over 50% damage reduction, so before any percentage increases that's cut down to 1470.
We'll assume you have enough health over her for LDR percentage to take full effect, with the 15% bonus physical damage increase that number goes up to 1,691 with a full channel. Adding the rest of masteries and the 7% armor pen we can safely call it anywhere from 1800-1900 damage.
I didn't do the math on the last mastery part, so feel free to correct any numbers after the LDR bonus.
This is where it's not adding up, does does a MF have 280 AD with those items? 3 infernals by the time she has 2 1/2 items?