Treantwood, Dhaos' Castle, and if you're powerful enough, the lower floors of the Morlia Mineshaft (not for the faint of heart).

Quote Originally Posted by sephirothishere
dont worry bout it....ill jus go to morlia mineshaft extension...sounds liek its got some hardy monsters....
If you see a monster named 'Land Urchin' (GBA) or 'Urchin' (SFC), RUN! IT KILLS ANYTHING IT TOUCHES. DO NOT WASTE TIME TRYING TO KILL IT, AS THERE'S ONLY ONE WAY TO DESTROY IT AND IT ISN'T WORTH IT ANYWAY. RUN. YOU WILL DIE IF YOU TRY TO ENGAGE IT. RUN. KEEP RUNNING? ARE YOU SAFE? HAVE YOU ESCAPED BATTLE YET?!

It's also very worthwhile to pay attention to the names of enemies when you enter a battle if you have the encounters set off so you don't start going into a Distortion Chaos Blade combo only to be smited to death by the Land Urchin.

And good luck with Morlia! It can be very frustrating (Land Urchin surprise), but it's a very nifty bonus dungeon. If you're playing the GBA/PS1 version, there are about five more floors with even more treasure, and a boss at the end.