Yea, people don't realize just how important that ult is for her burst in a fight. Without her ult the only way she can get anything resembling burst out in a fight is a very luck driven triple hit combo where she is standing next to a tentacle, and the enemy is coming strait at her + tentacle, and she lands EQW(on champ, not spirit) for 3 hits of her tentacle on both them and the spirit.
Seriously, there are two major actions against Illaoi you can't do against her without just giving her free gold.
1v2/3 her in lane post 6 as melees. Seriously, very stupid move. Her kit rewards her for having enemies around, fewer enemies means she does less damage. Bonus points to her if you guys are retarded enough to fight her when she has like 3 tentacles around her and has already marked one of you as vessels.
Let her build up her nest of tentacles freely. Kill the damn things as they spawn if you can afford to. Don't just let her spend her level 1 sitting in lane building up 3-4 tentacles that just give her dominance at level 2. If she is building up a nest and you can't contest her about it, at the very least don't follow her into the damned thing to fight her. Drag her out of her comfort zone (Her tentacle nest) and she loses a LOT of power.
If you do have to fight her, and she has ult, try to fight her in the middle of a lane/open area, where she has no nearby walls. If she ults, without a wall it tends to land the newly spawned tentacles outside of it's radius very heavily, more than enough to not be a threat in the fight unless you run to it. This can provide a degree of safety.