Illaoi is a feral and territorialist champion so play her as such, she shouldnt roam and her strenght is in keeping the lane oponent from farming (she is awesome against nasus), the area you place your tentacles is your domain so do not allow any enemy to enter it without punishment, use ur Q and W to poke whoever trespasses into your territory.

this build can be used in both top and mid

first thing to do is place 2/3 tentacles in lane before going to help the jungler, punish your oponent with a Q whenever he goes in to kill a minion, it will help you mark your territory, your W costs less mana and got less cd so you can use that too if there are tentacles nearby, you will end up pushing your lane so keep wards around you so you wont get caught of guard by the jungler, if the jungler goes to lane and you know you can beat you lane oponent pull the jungler's soul near a tentacle and get a few hits in his face, remember NEVER FOCUS THE SOUL, always attack the nearby champions and if possible hit both the soul and the champ with your Q and wall tentacle using W.

just play with your team now or if everyone is doing well push your lane and destroy the turrets, remember illaoi is not goot at roaming so do not wander too far from your tentacles as you are realy weak without them, if you want to fight against 2/3 people you may be able to do so alone but only if you are strong and they are not stronger than you, you may get a triple against enemies in the same strenght level but you will definately die if you try facing more than one stronger enemy, if you decide its time to ult remember to pull a soul before doing so and have as many champions as possible inside the area (including the soul), if you ult without a soul your tentacles will spawn but they will have nothing to do untill you use W so always have a soul in the ult area (that will also spawn an aditional tentacle for the soul) that way the tentacles will hit the soul (and everything nearby) as soon as they spawn, again never focus the soul no matter the situation, you may be able to kill a running enemy by hitting his soul if it got more remaining health than the champ, other than that just play with your team and remember that your W will leap to the target and it affects turrets so with iceborn gauntlet you can tear turrets/inhibitors/nexus a bit faster than normal.

that is it for this guide for now, i hope it helps you destroy your enemies, i will post a guide on support Illaoi latter, upvote it if you like it