The fact that he made it out of the volcano he either swam through lava or survived the eruption, and considering he made it out unscathed means that at the very least lava can't hurt him. Plus, the Fire Monster's fire, which was implied to be much hotter than lava, didn't hurt him, either.
First of all, Godzilla dove into the wall, burrowing his way out. Second of all, fire doesn't hurt to much when you can just sink into the water cause it was an aquatic battle.
Zilla Jr is just a sleeker, faster Godzilla from showa. He's impressive, sure, but has limited regeneration, cause as far as we know, Showa Goji never caught a common cold like ZillaJr. And he's taken toxins much more deadly then ZillaJr did.
He blasted through Q's nest.
A beam that can fire through buildings and poop, taking out a small rock ledge. Veeeeery impressive. What's more impressive? A similar sized ledge holding up Toho's 1984 Godzilla and it only broke away when explosives went off. Gee.