Some random suggestions: (1) David Pears pointed out that even if Kant's argument were wholly clear and wholly successful, which it is not, it could only show that existence is not an ordinary predicate, if it is a predicate. His view is that it is a predicate, just a very peculiar one; (2) There is also the view of the celebrated logician, mathematician and philosopher Bolzano, who writes in the Theory of Science ("Kinds of Propositions") that 'I take being [Sein] or actuality [Wirklichkeit] to be precisely what language makes it out to be, namely an attribute; whoever denies this confuses (I believe) actuality with substance. By substance I mean an actuality which is not an attribute of another actuality; hence I admit that we cannot truly predicate the putative abstractum of the substance (substantiality) of any object. For it is part of the concept of substance that there is no property of this kind. But it is not the same with actuality, which I consider to be a mere attribute, not only of substance itself but of each of its attributes, since every attribute of an actual thing is itself actual. And since every attribute of an object can be ascribed to it in a judgment of the form 'A has b', why not the attribute of actuality?' (3) There is a related argument deriving from Russell's Theory of Descriptions in my own Philosophical Propositions, despite the fact that Russell himself took the implication of the theory to be that the ontological argument is no good; (4) There is a defence of a stripped-down version of the ontological argument by the late Gary Matthews and Lynn Baker Rudder in Analysis for 2010.
Hello Philosophers! Can anyone defend the Ontological Argument against Kant's criticism that existence is not a predicate?