First of all, you're using Anselm's version of the argument. If you had done any research, you would have found that better versions have been developed. I think Plantinga's is best. It, by the way, does not assume existence is a property.

What you've done here seems to be taken directly from Dawkins' book.

The problem with it is that the Ontological Argument defines God as a Maximally Great Being. That is, the greatest possible being. A God who creates everything while not existing cannot be a Maximally Great Being, because he isn't a possible being.

(Also, I don't know why you talked about existence as a property, since this has not to do with your counter argument.)