robert casey wrote:

I vaguely remember a flying saucer turtle creature (with jet engines
where the feet should be) duking it out with Godzilla,

That's Gamera; he never fought Godzilla as he was from a different movie
studio:
http://en.wikipedia.org/wiki/Gamera

and trashing Tokyo in the process. :-)

He had a unfortunate habit of falling out of the sky onto things like
gasoline storage facilities and oil refineries.
If it could blow up, sooner or later Gamera would come tumbling down
onto it, generally in a unconscious condition after fighting some monster.

Pat