She is not that hard to deal with... Yes she has a LOT of damage but:
No mobility
Kited easily
Slow and predictable skillshots
litteraly NO CC
I mean, If your ADC gets
then she is probably done. Here are some good counters
: Counter pretty much any immobile tank due to her CC/mobility/self peel and her way too strong 12% true max health damage every three basic attack
with a blade of the ruined king, every passive proc will deal 12% current health and lucian has built in kite mechanics in his kit with his W and E
Kindred has everything to counter Illaoi, can kite, lot of %health damage and can even slow her if she manage to get to her.
It's alway fun to deal true damage and to kite tanks with Ahri, in the long run you always win.
Someone said % health damage?
Pretty much counters everyone. Mobile, %health true damage and sustain.