Alright, let’s me start this:
I believe Godzilla has the AP advantage with 526 GT compared to Rhea’s 409 GT. Not to big of a difference. Rhea has the advantage in that she has Flight and thus more maneuverable than Godzilla but Godzilla’s range far exceed that of Rhea’s but her Counterattck allow Rhea to attack back at any distance.
Rhea has Aurora Breath, which while requires minor prep, can not only ignore Godzilla’s durability, but also paralyze the creature and lower its stats by 10% for a short time.
In order for Godzilla to win, he have to take down Rhea about four times, and with each time, she gets more durable. With each defeat, she gains abilities like Death Blow and Wrath which increase her chance for big damage since Wrath increase her attack every time she initiate battle and Wrath grants her a much higher chance to crit when being attacked, which tripled her attacking power
Considering Ancient Dragonskin, she take half damage from each of Godzilla’s attacks. Then we have her Dragon-Scale Wall which, while only when her barrier is healthy, can reduce damage by 70%.
For now, I vote for the Child of the Goddess