There are three rules to laning against illaoi.
-Stay behind your creep wave - so you can farm the tentacles if you miss CS. They're 10g. It's not a bad idea. Now harassment or CS becomes a choice because the only way they attack is with W, E, or Ult, and dodging E isn't THAT hard, and the W is barely outside normal melee range.
-Pay attention to mana - she has mana issues early. Use that as you will. As long as you can dodge the skillshots and stay towards the middle of lane (because they spawn on terrain) you can dodge most other tentacle attacks.
-Always dodge E. Her E until she stacks CDR is something like 16 second CD. Paying attention to your timer becomes crucial.
is a good first back if you're having issues.
If you're going to ask for ganks, make sure you have some form of CC. CC is the achilles heel to illaoi. There's nothing she can do if she's CC'd.
Not an expert or anything. I've played maybe a dozen games max of illaoi, but I can pick up patterns easily on what problems are for champs I have SOME familiarity with in kits.
can deal with her.
Irelia is a good duelist, has that stun based on max health, and has some form of escape.
Gnar has mini gnar for kiting and poke, mega gnar has GREAT CC and a surprise element to his engagements if done right, and if properly timed.
Gangplank has his barrels for pressure from engagement, and can use his ult to at least get assists if he's behind anyways, or use it in lane when the jungler ganks, OR for a solo kill when she dives turret and gets slowed as well as turret aggro'd, on top of barrels. This is only after
. Before this? GP has the hardest time of these three.