Need to be lvl50 and done AF1. Go to Guslam in Upper Jueno and talk to him (Armor Shop there). This will unlock all coffer AFs. Meaning from this point you can do AF4, AF5, AF6 in any order. After this you need to get Eldime Coffer key. It's really a bitch to get. I suggest having monks and the higher lvl the better. Best kill Tomb Mages and Tomb Warriors, they are the easiest for keys. Once you get a key, find a coffer and you will get a key item. Take that item to Guslam. Then talk to Mithra in Tenshodo place by Lower Jueno AH. After that head over to Port Jueno and there is ??? by the Auction people there (where you go to chocobo stables). The ??? is on bunch of boxes, check it. After this you need to head over to Castle Zvahl Baileys. I forgot the zone location, but it's pretty easy to find. You will see 2 torches there by some kind of gate. Touch the torch and a Dark Spark will appear. Dark Spark is really easy to kill, it's a bomb. However, make sure you prevent self destruction, cause it can wipe out your mages. I personally would reccomend just a party of 52-55s for dark spark. But the key is much harder. Try to get some higher levels for that. Quoted straight from ffxi.allakhazam.com, pld af thread.