I, as a theist, also think that Craig's argument here is flawed. First of all, just because we can conceive of something, it doesn't mean that the thing is not necessary, but contingent. Say a coin, it must have a heads and a tails. I can conceive of just the heads part in my head. Doesn't mean that it's possible in reality. Furthermore, I can also conceive of a universe without god, but with each quark being necessary. Doesn't mean that God is not necessary.
I have this idea on how to prove to atheists the existence of God, namely:1. Tell atheists that for Christians, God in concept is first and foremost the creator cause of everything with a beginning.2. Tell them that the default status of things in the totality of reality is existence.3. Tell them that existence is divided into two kinds:(a) Necessary existence, for example, God in concept as first and foremost the creator cause of everything with a beginning, such an entity cannot not exist at all.(b) Contingent existence which depends on other entities to come into existence, for example, babies and roses4. The existence of contingent entities like babies and roses are ultimately the evidence to the existence of God, in concept as first and foremost the creator cause of everything with a beginning.5. Wherefore God exists, in concept as first and foremost the creator cause of everything with a beginning, and the proof is the evidence of everything with a beginning which are contingent, like for example, Babies and roses,Now, we can all Christians sit back and await to witness how atheists will react to the proof of God existing, in the step by step argument above.