BLU is a good DD for this with any support (RDM, WHM, SCH, SMN, DNC, etc). Cocoon for hits that slip through, Headbutt spam, Flashga as needed all make this easier. Be careful not to spend much time casting DD spells or you will die before he does. Ni is good; Ichi is too slow.
Went 1/7 on this guy with TH4 on two of those kills (without drop). Drop rate is thus fairly low and TH is recommended. Repop seems to be 3 to 6 hours (took me 3 days of camping to get this). I killed the 4 demons regularly under the assumption that one of them was the PH.
You can hunt other demon NM's between pops and farm NIN scrolls to alleviate some boredom, or just do what I did and get a good book...
Went 1/3 on this thing with a THF friend. It's a good idea to clear the room out and sort of kite the Marquis around a bit. Moving continuously will severely hamper his ability to land a quadrastrike. He has a potent regain effect that seems to increase as his HP decreases, and also has an add-effect of TP drain.
As for the drop, I've been full-timing it on DRG. It's no black tathlum, but it does have almost as much ACC and STR+1.
This NM does not play nice. He uses Quadrastrike for 400~1000 on a full evasion thf. Be prepared to take one to the face. Also seems to be a long lotto pop, anywhere from 1hr 30 mins to 4hr 30 mins.
Dropped from Marquis Sabnock, in Castle Zvahl Baileys. It can pop off of either of the demons on the close side of the room. Can be tanked by MNK/NIN, however Counterstance is a death sentence. Walking backwards at all times while locked on seems to be the best strategy, recasting shadows only when he is immobilized for a few seconds after he attempts to use Quadrastrike. Easy duo MNK and WHM.