I hope you've bought some Elixirs from the Phantom Village into the bargain. Really a lot of the bosses within The Void, like most other bosses in the game, will have some unique quirk that makes them easy to kill. Unfortunately the only way to know these is through intense experimentation or reading up in guides. Here are a few general rules of thumb;

- If you're not planning on heading back out then abuse GilToss like it's going out of fashion
- Run/Flee/Smoke from any random encounters if possible to save your HP/MP resources
- The last section made of crystal has a lot of high AP enemies. If you have a supply of cottages then use the last save point as a "base" and use up all of your cabins. Have everyone master Summoner at least to level 5 first to get Bahamut usable on different classes, and then keep using that. Movers especially, if killed quickly enough, give heaps of AP. If you master only one class, make it the Archer/Ranger