Ok, first, I hope you have made a seperate save file before you went to the dark world, otherwise if you don't have all the stuff that I reccomend, you could be in some serious trouble and you might have to start at an earlier save point if possible. So, before you go into the dark world, have all of this.

-Be at LEAST level 50, higher the better.
-Have the Ninja and Sage jobs
-Have a crapload of healing/revival items to get you through. (max out your inventory
-Have all the best spells, like Bahamut, Leviathan, Life2, etc.
-Make sure you have some ribbons equipped on your characters. I know you can have at least two so far, but one might let you get by.

Before you even take you have to take down the 4 bosses that guard the crystals first, (Cerberus, Echidna, 2 Headed Dragon and Ahriman) they will allow you to actually KILL the final boss, otherwise she will be invincible once again. Just to let you know, these four bosses you have to fight are no cakewalk at all. Cerberus can do some big lightning damage, plus his physical attack can petrify characters. Echidna has many ways to instantly kill characters, plus her Quake/Flare magics do a lot of damage to your whole party. 2 Headed Dragon can pretty much 1-2 hit any character on your team, the beast has a shiteload of strength. Ahriman's Meteor can do 3000+ damage to your whole team.

Basically, you need to be stocked up on plenty of items to get you through the Sylx Tower and beyond, because after that there is no resting point for your party. If you aren't prepared, then you're pretty much out of luck and you might need to restart to an eariler save point or even the whole game if you didn't back up your saves. Good Luck!