Well, I'm not sure exactly what kind of advice you're looking for, but I'd be willing to help with anything I can.

Do you need help getting in? Beat the game up until the final dungeon, then head to Warjilosntohawhatever (the city in the southeast corner of the map) and visit the Bar to unlock the Deep Dungeon.

Do you need help winning the battles? If you're having a lot of trouble and want the easy way out, bring Orlandu. If you want my preferred method, bring a handful of Chemists and Archers along. The irregularities of the terrain in the Deep Dungeon swing the battle in favor of those who can attack at a distance, so you'll find bows and guns very handy. Since you often start with a massive elevation advantage, park your archers up there and snipe a bunch.

Do you need help finding items or exits? Go to gamefaqs and look for a Deep Dungeon guide.

If you can be more specific, I can give you more detailed advice.