I got with Illaoi from Silver 5 to Platin 5 in one Season just by practicing and testing whats best for my play style.
I think the biggest difference between mine and other guides is Death's Dance, I nearly never buy it and the reason is mana.
I prefer Iceborn instead of Death's Dance because I have mana problems every game and I love how Black Cleaver and Iceborn fit together.

First try tu have on your passive and sawn a tentacle where you need it. They are the biggest part of your hole kit and try to fight next to them.
Your E is the most important ability. Don't use it to often cause it cost you a lot of mana and the cd is very high at the beginning.
Only use it when you stand next to a tentacle or even better tow tentacles.
Once you hit level 6 you can truly start fighting, even 1 v 2.
Your ultimate can do a lot and change a hole teamfigth if you use it good.
The best way to win a fight against one or more enemy's ist to land a E on one of the carry's and ult flash into them.