@alextheboss: How? He would keep his strength, speed and durability just like a shrunk antman. Plus his hits would be way deadlier due to higher pressure. Shrinking him would make this an even worse stomp.
@nwgzsjuwhm96y2: If Godzilla really retains all of his stats while shrunk I guess antman can't beat him. But I'm not sure if Godzilla can really do anything while tiny. Giantman can just pick him up and try and toss him as far as possible (shrunken objects don't retain their weight). It might be a stalemate.