Looking at the Illaoi, her going "Super Tank" is false as all hell.
She went bruiser Illaoi, shred all of your armor with Black Cleaver (her Tentacles are physical damage so each slap procs it too), she has Death's Dance which was healing her each time she auto'd or W'd, she has Trinity Force + Sterak's which gives her survivability and high damage (especially with her ult active cutting her W's cooldown drastically), Thornmail to tank hits from you, and Spirit Visage to increase the healing from her tentacles and Death's Dance.
As you might notice, she's missing boots. This also means that she is missing the ability to move fast before hitting anyone, thus should be easily avoidable at all times.
In addition, I notice that you failed to buy Thornmail, which is a key tank item vs Illaoi. You buy Thornmail >> she right clicks you once >> she can no longer heal effectively even with Spirit Visage.
Not to mention that, as a Nasus, your Wither is a major counter-CC to her because of how powerful of a slow it is, on top of her lack of boots as I mentioned before.
Honestly speaking, you losing to this Illaoi was not Illaoi's fault at all, it was your own fault for letting her do as she pleased. My guess is that you panic Withered everything but Illaoi, allowing her to get into your faces and fight you, instead of using it on her so she would become a non-factor in every single fight.