passive: 10% reduced damage from 1st champion attacking her, 20% reduced from 2nd champion attacking her, etc
active: 1. 40% tenacity vs marked champion, 60% tenacity vs 2nd champion attacking her, etc
2. increases damage to marked champion
I think this would preserve the tanky assassin/bruiser theme,
It's getting removed.
From a coding perspective, it seems it would be difficult to set that up when multiple people attack her in a small window.
A good example is 
's spell shield, it has a flat duration for how long it lasts, and then it has a much smaller duration in which a spell that hits in that duration will get rid of the shield. The smaller duration is about .25 seconds if I remember correctly. Currently if she is hit with 2 spells that reach her within .25 seconds of each other, she will block both of them. Technically, the spell shield should only block 1 spell, but the coding has that small window for reasons only Riot knows, but it will end up blocking 2 instead.
perhaps more coding friendly options then:
1, upon activation, if she's hit by physical damage, she's immune to magic damage for the duration (and viceversa). 2. passive: detect predominant type of damage in last x seconds, and decrease damage of that type for y seconds by z%
