"The third consecutive attack or spell hits against the same target will deal additional true damage plus a percentage of the target's maximum health as true damage (up to 8%)."

Question: How do you deal with this as a tank?

Build armor? It's true damage.

Build health? It's % health true damage.

Build damage? Regular AD will wreck you.

Am I missing something?

Debuff her attack speed with Frozen Heart/Randuin's Omen.

Realize that even if you're stacking armor and HP, it'll still be less 20% of her total damage output.

frozen heart, nasus, radiums.

If you're a disruption tank, ask your team why she is not dying in <1 second,
If you're a damage tank, your base damage is high and you have some AD/AP from RoA or passives. Use your gap-closer and kill her.

It still helps to get more health despite the true damage percent thing. A squishy without extra health would die in three hits to her end game.

Focus her=D

Malphites E ability or anything to reduce attack-speed while focusing her.