This unknown Earth god had spent a lot of energy destroying this continent and killing everyone inside of it, and in addition to the Tribulation they were currently undergoing, it would be amazing if they had a fraction of their energy stores left, and under the crushing power of the Tribulation, they was going to be greatly distracted, this would give Rowan the brief window that would aid him in damaging the corporeal body of this Earth god to the extent that he might exhaust the last drop of Aether and Essence in their body.
In fact, he might not need to even reach that point if he could destabilize the mental state of the Earth god to the extent that they could no longer handle their Tribulation effectively, the backlash they would receive would be able to kill them, and all of this would have to be done without him making a single mistake, because the present gap between this body and the Earth god was so massive that only a slight error on his part and he would be vaporized.