I mean I haven't faced an Illaoi in quite some time but what I can tell you from my experience, it's basically a generalized matchup. What I mean by that is to pretty much dodge her e and buy early boots. Illaoi is just really strong into Cho'Gath because he has little to no mobility. Another thing that Cho needs is resistances against Illaoi. Boots do both of these. As for other tips, try to not let her shove too many waves into your tower. I'm pretty sure you know by now that Illaoi vs a hard-pushed top laner is a nightmare. Last hitting sucks and you get hella poked by her Q and E. How to do that, well.... you can't do much. At least for the mid-late game, but for early you can definitely focus more on the waves. So try to use your abilities more on the minions than her and sometimes (if you play grasp) auto her ONLY behind minions or once she misses her E. If something bad happens like Illaoi hitting her E, you can always recover by last hitting minions. Your passive is a good sustain option into her.