I can't seriously think that the circle for Xin Zhao's ult making him invulnerable to damage from outside of it is stupid, right??
Seriously, I know that getting rid of that effect would render it useless, but maybe you could reconsider where damage sources come from?
Illaoi's ultimate should be able to damage him still. Maybe have it so Illaoi still has to be in the circle for it, I don't care, just... right now it's pretty unbalanced, directly in Xin's favour.