The Black Coffin can be a fairly simple fight. Bring one or two BLM or /BLM for sleep 2/ sleepga. Put the first wave to sleep and try to kill them one at a time (you can either assist gessho or let him tank one on his own).
For the second wave, you need only to defeat the captain to win the fight, but it can be advisable to kill a few crew members as well or the mages can find themselves in a bad spot with aggro. Overall not too hard with patience and planning.
And remember each member needs to get the ephramadian gold coin each time they run this battle, even if they've beaten it before.