1. Whatever is conceivable C is possibly caused by an existing C's impression. (Premise)
2. A perfect being is conceivable. (Premise)
3. Hence, the conception of a perfect being is possibly caused by a perfect being's impression. (From 1 and 2)
4. Hence, a perfect being possibly exists. (Implied by 3)
5. Therefore, a perfect being exists. (From 4 and S5)
Besides the usual unpackaging of (4), this argument is defensible. Descartes held that in order to conceive of something, there had to be an impression of that conception caused by the entity being conceived. That's a bit of a mouthful, but the idea can be illustrated by connotation, as opposed to strict denotation. The reason I can conceive of a horse is because I have seen an existing horse. Even imaginary entities, such as a unicorn, are just an amalgamation of existing things, e.g. a horse (which exists) and a horn (which exists on other animals).
Now, instead of contending that Descartes' strong principle is correct, we can actually weaken it and still arrive at the same conclusion. If it is even possible for a conception to be caused in the manner described above, it follows that a perfect being (omnipotent, omniscient, and morally perfect in every possible world) possibly exists. All that's left is a defense of the rather uncontroversial S5 axiom, and we have a successful ontological argument for God's existence.
Of course, I don't expect anyone to be persuaded by this argument if they're not persuaded by similar ones, e.g. the modal third way, that have arguably even more modest premises.