Castle Oztroja isn't that hard, but you're going to need someone with a movement speed enhancement in order to clear the doors at the top. You will also need invisibility for the section after the 4-switch door. All other parts can be accessed without a problem, but I'd take note of easy preys in the corner torch room prior to the second statue and the leech pool after the 4-switch door.
Take with you...
1. Some way of becoming invisible (WHM sub, a WHM, a RDM, Tonko, or Prism Powders).
2. Instant Warp Scroll or method of warping out (for the exitless area at the end).
3. A movement speed enhancement (Strider Boots, Ninja AF Boots, or Flee) OR A PARTNER to open the final door for you. You'll usually find a THF up there hunting coffers anyway for their AF or Astral Rings, you can ask him to light the torch for you to open the final door.
4. Have "Rank 5" so you can open the doors that require seals. If not needed here, you will need it in Beadeaux.
There are three statues that you're going to have to visit in order to get the passwords. These passwords change every in-game day, so if you don't know your way around the castle, I recommend starting this at 0:00 in-game time. If you are very familiar with the route, you can do it in under 30 mins (or 12:00 in-game time).
Each statue room will be behind a torch door and will give you a four character password for use on the top level.
The first one is on the ground floor very close to the entrance. It's located at H-8 of the following map.
http://www.speakeasy.org/~sticks/ffxi/trea..._Oztroja_01.gif
The second one is very close to the torch door you had to visit when you did the Mission 4 quests for Rank 5, but instead of going to the torch door, you'll want to go East into the big room. The big room contains an Ooze pond in the center. The statue room is to the SE of that room behind the torch door. It'll be located in H-9 of the following map.
http://www.speakeasy.org/~sticks/ffxi/trea..._Oztroja_07.gif
The third statue is on the same floor as the bridge leading to the 4 switch door. Instead of going immediately to the bridge across just go North a few more seconds and you'll get to the final statue room. It'll be located at the NW corner of I-8 on the following map.
http://www.speakeasy.org/~sticks/ffxi/trea..._Oztroja_02.gif
Once you have all three passwords written down, the only thing you have to do is get to the top level to enter them. Go to the 4 switch door (SW corner of G-8) and manipulate the switches until you finally get the door open. I believe that the switch combination changes each day as well, so it may take a little while. When you do get the door open, you should quickly bolt to the other side and immediately turn invisible. From this point on it's fairly easy, all you have to do is get to the top floor and reach the door all the way North (it'll be a big heavy door). You can have a THF light the torch, hit Flee, and hit Hide and meet you in the room to the North or you can have your partner light it if he doesn't need it (or have one of the many THF's in there light it for you or your group).
Once you are in the North room all you have to do is enter the three passwords you received earlier, the floor will drop out from under you if you entered them correctly(if anyone else is with you, have them stand on the trap door as well). You will be in an exitless area with Yagudos. Make sure to keep invisibility up when you are going for the ??? spot. You'll see it on the center platform by a Yagudo High Preist.
DO NOT GET IN THE LINE OF SIGHT OF THE YAGUDO HIGH PREIST, YAGUDO AVATAR, OR YAGUDO TEMPLAR. They can see through invisibility, so your best bet is to chill out and find the perfect time to take invisibility off and click on the ???. Just do it when their backs are turned and you'll pick up the Castle Oztroja Crest. Once you've got it, get the hell out of there =).
For nearly the same info, you can find the same quest written out on Allakhazam.
http://ffxi.allakhazam.com/db/quests.html?realm=&fquest=282