Don't stand where her Q can hit you? It half hp's you because she gets her passive, duskblade proc and it crits from bouncing off a minion it kills, also procs dark harvest if it does enough damage. All of that is increased by the lethality and AD of Duskblade.
It's a really big item powerspike. That's what you're missing here. The enemy MF knows her item powerspike and you don't know hers, or yours...
You bought boots of swiftness on Brand. This is really bad because Brand has high base damage and relatively low AP scalings for a mage (that, with his low mana costs, is why he's a support). So you have to buy magic pen early.
give you the highest % increase in post-mitigation damage for the lowest cost early on. When you get to
on your second or 3rd back, you turn into a damage beast. Then you buy Rylai's and Liandry's, Morello/Void elixir if you get that far.
Hope this helps in your fiery adventures :^)