Human nature can mean man's essential or created nature; it can mean man's existential or estranged nature; and it can mean man's nature in the ambiguous unity of the two others. ... Under these circumstances it is imperative to dismiss altogether the term "human nature" in relation to the Christ and replace it by a descriptions of the dynamics of life - as we have tried to do.
Systematic Theology II, 147.
Tillich has a similar criticism of the term "divine nature". His point is that these terms, as we normally use them are totally inadequate when it comes to describing Christ today. It is important to stress that Tillich does not reject the Calchedonic theology, merely it's formula. In fact he defends the divinty of Christ against liberals such as von Harnack. But on the other hand, not to recognize that the traditional formulas are problematic is to retreat from the claim of a gospel relevant for today into a Christian sub-culture.
For more on Tillich's Christology, check out this post by "WTM" at "der evangelische Theologe".