Like HOLY FLARK THE DAMAGE SHE HAS US STUPID, HER E IS STUPID, HER Q IS STUPID, HER W IS STUPID, HER ULT IS STUPID, THE LIFE STEAL SHE GETS FROM HITTING HER ULT COMBOED WITH W IS STUPID, Rant over.
How does one counterplay her, I need genuine tips other than "BAN HER HURR DURR". I want to experience the sweet satisfaction of ripping her ass a new one with her own tentacles. Please help me
"i fought a champion for the first time and didnt know how to play against it so i got wrecked bwaaaaaa"
Walk out of her Q. Litterally. Its so easy to do. Counterpick. You dont pick teemo into darius, do you? Kill her tentacles.
DONT LET HER GET A 5 MAN ULT. Jesus i see it so often that illaoi snowballs (Ive done it too) and people just forget about her ult and dont even bother NOT clumping up and letting her get that sweet 5man ult.
illaoi is garbage.
you do something called sidestepping her extremely slow tentacles.
if she hits you with e hit her back so you dont become a vessal
and ofc build 
I picked her up some when she came out and played against her some. Here's what I've learned:
So a lot of what I'm going to tell you is about the tentacles. From what I can tell, the #1 rule is: AVOID THE TENTACLE STRIKES. If you can do that, you nullify a lot of her damage. Because the strikes do damage and heal her, it's very hard to win a trade while being hit by them, especially if there's more than one hitting you. Her Q is a good harrass tool to deny CS, but it will shove the lane hard and it's not terribly hard to dodge.
A big thing that gives people trouble in lane against Illaoi is her E. Being hit with her E makes it very difficult to fight her, and leaves you with some pretty rough choices. I'm not going to tell you don't get hit because it's obvious, patronizing, and highly situation-dependent. I will tell you to bait it out every chance you get. While her E is on cooldown, her ability to fight you is greatly reduced. Tentacles will swing at spirits and vessels. The only other way she has to force them to attack is with her W, which (except during ult) she won't have more than once per trade. This means that landing her E will oftentimes double her damage output. Furthermore, Illaoi will look to land the E near an existing tentacle in order to get the best damage out of it. A great way to bait the E is by walking towards a tentacle as though you're about to auto attack it, and turn back when she's about to throw out her E. It's a dangerous game because it's a rough spot to get hit, but rewarding. If she misses, you can kill the tentacle and try to land some damage on her. Even if she W's to you, all you have to do is move out of the way of the strike and she's left without much to hit you with.
Sometimes, though, you get hit with the E. Here's some options:
That just leaves her R. Here's two ways to counter her R:
No but really. If those tentacles a keep smashing, she can 1 v 2 or 1 v 3 pretty regularly. Because her ult will spawn more tentacles for more people it hits, you may take more damage with teammates nearby. What she can't do is move the tentacles, and she has pretty much no hard CC. So just back up, wait for the ult duration to be over, and then go back to whatever you were doing.
Hope this helps. Try to see past how frustrating it is. If all you ever think is "wow this bullshit champ is so op" you won't learn anything. Doesn't mean she's not... overtuned though ;)
EDIT why all the downvotes. He's asking for advice
Play 
.
Problem solved.
illaoi is like a donger except she has plenty of advantage that donger doesnt have, like she can build a blackcleaver then full tank but still deal a load of dmg, she has free heal from her tenticle, and her regular passive tenticles cost nothing.
movespeed quints or early boots, now you beat her
she gets absolutely wrecked by 2 simple things. range and mobility.
everyone on that list has what it takes to stomp illaoi in lane.
tips: dodge every tentacle that you possibly can, ESPECIALLY her E. clear her tentacles from the sides of the lane at every opportunity, this is where range shines against her, as it allows you to take the tentacles in complete safety almost immediately after they spawn. remember that she lacks both mobility and CC, so simply walking away from her is always an option, and this option should be abused every time she ults, as her ult is where a huge amount of her power and sustain come from. make sure to remind your team to stay away from her ult (before they get hit by it, not after) and to just get away from her when she ults.
it helps to think of her ult as being similar to a nunu ult, you do not want to be inside it. hope my tips helped c:
