Ultima-Griever
Sorceress
Jesus Christ. Whoever came up with the idea of creating the Great Crystal was insane and wanted the players to go insane, too. The only time I had no difficulty going through it was on the first time I got there: of course, because there is only one way you can go: to the Crystal Core. But when you go upwards, either to get Ultima and/or defeat Omega Mark XII, it's nearly impossible to go through without a freaking map. I don't know if it was only me, but I was stuck at the Great Crystal for nearly 4 hours and a half, completely lost, until I stumbled upon Ultima. Thankfully my party was in a high level, mainly due to the monsters inside the crystal, and I defeated her in a single fight. Then again, to find Omega Mark XII - another torture, and I was even looking at a map. That background music is annoying and so is the noise of the party running around. I wish I can never ever do anything there again, I don't even feel encouraged to find the Zodiac Escutcheon there. ):
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i will be getting all the bonus stuff so i guess this place is where im going to spend many many hours, thanks fr the heads up 

It's horrible and you'll be wanting to get out as fast as possible. If you like it, you can just go back for Ultima (yet another useless esper).