Godzilla's proven to withstand temperatures far in excess of 1,200C. Fire Mirror, Thunder Control, and magma seas didn't melt him. Those do not include intense but brief point heating from such things as explosive warheads and beam cannon.
Given the canon examples we've seen, we can not confidently say that Godzilla even has a meltdown temperature normally. We can only say that Godzilla had an internal temp. limit of 1,200C after the island incident early in "GvsD."
Similarly, we needn't assume Godzilla couldn't stand in "our" world. The radiovore strong-cell idea I put forth adequately addresses the square/cube problem, as well as explaining Godzilla's regeneration and his bizarre feeding habits.