For all of you who somehow never heard about Illaoi's E, let me inform you so you stop feeding.

You cannot, whatsoever, back while a vessel. You may be low on health. But you cannot back until you stop being a vessel. dodge all the spawned tentacles. They are easy to dodge since you know when and where they will strike (at the position you were at when they start the swing animation). Better yet, dont get hooked in the first place. It is a very tellable ability. She needs a direct path to the target (you). And it is a skill shot.

Illaoi needs her tentacles up to deal damage. They are like 80% of her output. SO dont leave them just sitting there dormant. Clear them. If you know her E is on CD (it has a very long one at that 20s-18s-16s-14s-12s) go ahead and clear them, even if it costs you 1 minion. They only spawn every 20-12s (based on level) and only one on a surface in a given distance (besides her ult spawned ones, but those are untargetable anyways).

Pick range or massive sustain or some form of hard cc/silence against her for an easy win in lane. She has no kill threat unless she can get right on top of you or snag you with her E enough times. Good counters are/can be Bad matchups are/can be