The only save spot in the Lunar Subterrean is located on Floor B7. It's the first room of a row of three.
The second room has two Ribbons in it. The third you can get Kain's Ultimate Weapon from (in the main game anyway). You've got to fight Plague, who can be a tough battle, as he casts Doom on the whole party. The best thing to do is, cast Slow on yourself, and kill of one or two characters before you fight him, then revive them one at a time as the counter gets to around 3. The rest of the bosses shoulden't be to tough.