She's a juggernaut. She can't get to the backline easely, since she has no gap closer. If she flashes onto you (the adc), either you are overextended or your frontline is stupid. and if she flashes onto you, use your own to get away.
Then, kiting her does wonders. She has no CC and no way of getting to you. The only thing she can do is W onto you IF she manages to get in aa range or to use her E.
When she used her E on you, fight back. Don't let her do free damage to you. Every time you do damage to her, you reduce the duration of the spirit by one second.
Her E is also quite easy to dodge. The Hitbox is quite narrow and it's a slow projectile, so unless she uses it point blank, you have a good chance of dodging it. And the CD on hit is so long that she's won't be able to use it many times in a fight.
Items like
will help you fight her.
If she ult on you, disengage ASAP. That area is now a no-no zone as long as she's there. Illaoi's main source of damage come from her tentacles. Without it, her damage is quite underwhelming compared to other juggernauts