I have to disagree with having nothing to do about being behind. Illaoi's zone control and aoe damage make farming under turret much easier than other top laners. I got pushed up by a fizz and took about 3/4 of 2 full waves while baiting him with damage and hooking his spirit into tower range. He was a vessel in an instant and I got major gold.
I've also successfully out-dueled Fiora w/ignite and Darius. You have to make them play into your game. Make sure your tentacles are in bushes when you can and punish them for trying to destroy them. Also, use your W on minions while they're last hitting, it's still an aoe and they'll either eat it or it's a successful denial.
If you can get them up to turret, I like to focus on nabbing an E while they're on defense. If you can make them a vessel there, then they can't focus on defending their turret. Just be ready for the jungle. Usually your tentacles will be near you, on their turret if you position right, so they'll have to come into your sight for at least one of their three tentacle kills. Couple that with the fact that you get a flash of vision on them when a tentacle spawns on them anywhere while they're debuffed and a nicely warded river and tri bush and you get an almost uninterrupted tower siege. Just watch your back.
I usually like to build Doran's Blade>Tiamat>Specter's Cowl/Warden's Mail. Finish Titanic Hydra/Spirit Visage/Randuin's then build situational damage from there. I like Trinity Force and Black Cleaver.
For her ult, I like to use it for massive COUNTER initiation. I use my Q and E to harass, especially in teamfights. Clear the minion waves with your team and your Q and have a clear lane to push the enemy team back by tossing out your E. If you snag someone with your whole team nearby, that's a guaranteed vessel and effort free backline harass while the vessel runs around their team spawning vessels, or they venture alone to spare the team the damage. The kicker here is if you can keep the lane clear they'll be one man down, do your best to land ANOTHER E. Get another vessel and tap out another team member. Going back to what I said about laning, most of the tentacles will be near you. At this point you can save your ult for stopping whatever they're gonna throw at you to counter. So watch the fight and stay in range to get to your team and ult, but focus on denying your vessels the tentacle kills they so desperately need to get back into the fight. Let your team do the work from here while you shut out at least 1 of their teammates. (Ideal situation, yes. But it's starting to become a pattern in my games. Sticking to it with obvious flexibility is working well.)
Lot's more to say, but it's Thanksgiving and this was way longer than intended, lol.
