No, actually that's true independent of Christianity.
You won't find terms like "Maximally Great Being" in Scripture. Thankfully though you will find that the Biblical portrayal of God does indeed align with a MGB, which is good as it lends credit to the reliability and truthfulness of God as outlined in Scripture.
But philosophically speaking, if God exists, then He is by nature a Maximally Great Being. There can be no conceivably greater being than God. If there were, then that being would be God. God, by nature, if He exists, is the greatest conceivable being.
The greatest conceivable being cannot be killed. Therefore, God, if He exists, cannot be killed.
What the enemy does or doesn't keep in his heart is unknown to me.