Generally, you can poke with Q, and if they are low enough, go all in with an E, W, Q, W. Other than this, play safe and farm. Late Game you shine.

All in's with your ult, look to get Rift Herald, and split top. You can fight and kill 2 enemies easily, and will usually walk away from 3. If they disengage after your ult, go ahead and walk back to your tower, they know how to fight Illaoi. Just try to bring some people top and tell your team to push mid.

Tentacle up baron and dragons, and ask the enemy team to fight. You will get a double kill and the baron/dragon. Split push if you got RH, and group up if not. If you are splitting, try to make 3 or more come top. This will give your team plenty of time to take other towers, and you can win 1v3s, or at least take a few down with you. If you got fed in mid game, go ahead and play extra aggressive. If not, prep up your tentacles and land your E on the target with the most damage. They will /all "Nice hentai" and you will get LP.

I dont know what else to throw on here atm, so if theres something I left out, go ahead and ask me in the comments.