Mystical Facts About Illaoi's Test of Spirit
If you auto attack Illaoi, the duration of the spirit pulled from you will be decreased and you might get it back before you get turned into a vessel.
If you get turned into a vessel, you can avoid taking damage from tentacle spawns if you stay near a spawned tentacle and **NOT ** destroy it. Not destroying tentacles and being near their spawns makes it that you can't spawn any new tentacles and as such no tentacles will lash out at you. Of course this point is moot if you are still battling Illaoi and she will tackle you with a harsh lesson, but it is pretty useful information when you are by yourself and trying to get back. This will help you from dying alone to tentacle spawns.
Short version: Auto attack Illaoi to fight for your spirit back and don't kill tentacle spawns when you or a teammate are a vessel!