Once you gain access to the Ko-Kan Horay, you can battle the monsters there as much as desired. Use a little party experience to put everyone up to around level 10 so that you can easily pass through the first few levels. Make sure you have a lot of good skills. The following are recommended for each character:

Ryu:
Legend Blade +1 (can be purchased from Faerie Village, unless you have better)
Level 1: Slice, Side Slash, Dent Armor
Level 2: Discharge, Vert Slash, Spiritcharge
Level 3: Roundsabre, Circle Stage, Great Smash

Nina:
Wizard Wand +2 (Faerie Villige, unless better)
Level 1: Flare, Frost, Jolt
Level 2: Fireblast, Iceblast, Lightning
Level 3: Inferno, Blizzard, Lag Valhalla

Lin:
Gernade +1 (Faerie Village, unless better)
Level 1: Hey Hey!, Hows that?, There!
Level 2: Stay here!, Thats it! ,Take that!
Level 3: Shatter!, Blow up!, Let's Dance!

Also, make sure to have about five pages in your backpack and stock them with around 20 Aid Kits, 10 Med Kits, 10 Tonics, and whatever else desired. You should not have to use all of them, but it is good to be safe. Buy the armor from Faerie Village that prevents all status effects and equip it to everyone. Make everyone have a shield with an open slot and place Feint in each one. If you make it down to around level 20 to 25, exit and repeat. You should gain around 8,500 party experience each run through. It should only cost about 1,500 to bring everyone up to level 10 at the beginning. You can go further if you think you can make it; however, try to save up your party experience. Once you have about 120,000 party experience or more, you should be able to go all the way through the dungeon.