What counters True Damage? Pure HP.
Nothing, however, counters %Max HP True Damage... equally efficient vs. every Champion it faces.
Now, that's just the Counter to Base True Damage... but the real counter is in how to counter the Champion. The ONLY League that Fiora blew up in was NA... everyone else held her either in check or curb stomped her.
So with Fiora it was a flaw in how NA was playing against her, until Huni figured out Quinn is a Skill Match Up vs. her and whoever found out that Malphite and Maokai are two tough hombres vs. her in spite of her Max Health Percentile True Damage.
Yi? Chain him down and burst him. Tank Yi? Chain him down and Sustain him.
Same for Vayne. Just one Hard CC is all it takes, and POP goes the Vayne.
Cho? Dude, are you mad man? Why are you trying to 1v1 a Cho? HE EATS WORLDS. ENTIRE PLANETS. Are you greater than an entire planet? So CC his big, slow azz and dump a ton of Ranged Damage into his body until you and your Allies DEFEAT THE EATER OF WORLDS! HAHAHAHAHAHAHA MWAHAHAHAHAHAHAHAH! Seriously, only certain Champions are fit to Solo a Cho'lo. Use 'Team'.
Vel. Dodge. Then destroy.