Premise 1: Being real is more perfect than being unreal.
Premise 2: God is the most perfect.
You can certainly imagine a thing than which nothing more perfect can be imagined, because if you understand the phrase “most perfect being”, you have already imagined such a thing. If this thing does not exist in reality, you can then imagine it existing in reality, which is more perfect. And it contradicts the assumption that the previous thing is the most perfect that you can imagine. Hence, in reality there exists a being than which nothing more perfect can be imagined, and it is God. Therefore, God exists.
Flaw: You can imagine a thing than which nothing more perfect can be imagined, and in your opinion, it exists in reality. Now that you have already imagined it existing in reality, you cannot imagine a more perfect thing. And imagining it exists in reality does not mean it truly exists in reality.