Just tried Promyvion-Mea tonight with a group of 16.
Like everyone else has said, first floor is a joke, second floor is a little more difficult.
It's that third floor that makes things complicated. If it weren't for this floor, you could easily get to the spire with 12 people. But this stinking Memory Receptacle and all it's Strays are very difficult to defeat even with 18 people. If anyone could work out a great strategy for this, please let me know.
4th floor, do your best to avoid aggroing anything, these guys are painful.
THE BOSS FIGHT:
Come over-prepared for this fight. This isn't the Rank 3 dragon battle, you can't waltz in without potions, drinks, food, etc. and expect to win.
He hits NASTY hard, terrible knockback attacks, and the one I loved the most was the "Impaler" attack. Did 446 damage to me. On the flip side, since I took so much damage, I lost all my Emnity so his focus was no longer on me. That kept me alive more than once.
This really is nothing more than a battle of attrition. If you have the right amount of food, drinks, etc, you should be able to outlast him.
If you want to win, COME PREPARED!
Also of note, BLM don't really need to waste their MP with nukes on this guy, it does jack for damage. Very high resistance. RNG is the nuker of choice in this fight. However, that doesn't leave BLM with nothing to do: they can still be backup healers.
Also, do not use the RNG 2 hour Eagle Eye Shot until it is down to it's last leg of health (below 15%). You will get aggro, and you will most likely die, and so will the rest of your party.
Hope some of this helps.