Anyone who attacks any metaphysical position using ontology would first have to justify the validity of ontology.
So I am not holding my breath for that.
When you boil it down, Materialism is defined functionally - that everything, including our minds, is ultimately governed by some combination of necessity and chance.
Idealism, on the other hand, must contend that mental things such as will, cognition and understanding are not reducible to a combination of necessity and chance, that they are things in and of themselves.
If I had to defend a position I would rather be defending the former, rather than the latter.