I am seeking metallo still and I was wondering if there is a way to fight endlessly in the dungeons without having to return to town after clearing them each time. Something like how you can fight endlessly in the field or does nothing like this exist currently?

There isn’t. 

Darn, well maybe they should implement one then :-).

It isn’t really a bad idea, actually. Though in most cases it isn’t too much trouble to go back from the town, is it? 

Only that it resets your charge for summoning a first one back to zero so you have to start over again. Also summoning a first one looking for starters is funny I think.