The modal ontological argument for God

[Sheldon]

Modal Ontological Argument
[Short form]

P1. If a god is possible it exists in every possible world
P2. If a god exists in every possible world then it would exist in this world as this world is possible
C. God exists

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My response to this argument is that just because God is logically possible it doesn't make God metaphysically possible. So we can just ask for a justification for why God is metaphysically possible.

P1 hinges on how one is defining a deity, I would need see possibility objectively demonstrated before I accept it

P2 Since I don't accept P1, this is just a circular reasoning fallacy. 

I can't accept the conclusion logically follows, since I don't believe the premises are true. 

Now try this, replace god with unicorn, and explain to me what the argument loses? All one need do is define anything in the way apologists arbitrarily define god, and the argument loses nothing.