Lethality Illaoi is hard to master and requires countless hours learning how tentacle setup works or just hitting E, without E you are nothing and so your ultimate means nothing. Just go lethality if they have a lot of squishes, or a couple of them you could farm kills and also you'll scale enough so you get Serylda and just dominate tanks overall.

First when the game starts, guessing you are playing middle just rush and place tentacles on the bot side ledge if you are blue side, or top side ledge if you are red side:And next tentacle would be:
Or you can just go with the enemy side's ledge as you did on the first image on your side.

This is very important in keeping dominance on the lane, if you manage to do this start leveling up with W to punish any move from the enemy. If you fail to do tentacle setup just go with Q.

The main combo is just Test of Spirit enemy with E and make them come closer to you so you can use ultimate casting a lot more tentacles and also sending 2x damage to the enemy, make sure to not die easily, or if you get killed easily just go for situational items like Armor or MR. It will send burst damage to the enemy team and if you have Serylda it will nullify their armor making you the strongest in the game.

This currently works for patch 14.1 since items are OP right now, in the last season and patches I always blamed champions but now it is items. Lethality Illaoi always worked and will always work but it's best season to dominate with your tentacles.