But she isn't fighting God, she's fighting the JLA. Just because the JLA had a member once that was God's angel doesn't mean God, or even his angels, get to participate in the fight.
If we play the "connections" game, the situation becomes even more ridiculous than it already is. If Zauriel gets to call up the Pax Dei, then who says that Jean Grey can't call up the Phoenix Corps shown at the end of Here Comes Tomorrow? Every single one of them is a potential Phoenix of a particular reality. If you thought Jean Grey having infinite power wasn't enough, how about an infinite amount of that infinite power?
Hell, why doesn't Xavier call up Dr. Strange, since they were both in the Illuminati? And then Dr. Strange can call up Eternity or the Living Tribunal?
There has to be set rules and set participants and set baselines powers for this fight, or else it just becomes a giant pissing contest of "Who can destroy the world faster?" The fact is that both of these teams, at the most powerful that they've ever been shown in any continuity, has members who can destroy the world several times over...and then recreate it again and again. By those guidelines it can't possibly end in anything but a draw.