I wouldn't consider him invicible, per se. I think (in his first form) he's just a spiritual manifestation of Zemus, and thus can't interact with the laws of physical reality in the normal sense. (i.e., he can't be sliced to pieces with a sword)

I guess it's possible that he really is just that strong, though. Using the Crystal apparently weakens and mutates him, making him vulnerable to destruction.

It can also be said that he is destruction incarnate itself, and is thus immune to all forms of destruction. :P