These mobs cannot be "sneak popped" to fight only the weapon - all three aggro on pop even with sneak.
You do not have to have Dark Knight set as your main job when you pop the NM in order to receive the helmet.
Recommended strategy is to have three different people tanking, one to tank the weapon, and two more to hold the victims, with a healer for support, along with other DDs. Pick the skeletons off as fast as possible, then have everyone focus on the weapon. Once the victims are down, it should be just a straight up fight. (see testimonials)
Only Gerwitz's Scythe must be defeated. If you wipe you can come back and may still get the Chaos Burgonet.
After you kill the three NMs, take the teleporter out and you will receive a cutscene and your reward.
Kyokusa's Video Guide
A Visual guide that will help you prepare for what is in store for you.
Jobs used:
WHM 75
MNK 75
THF 75
PLD 75
Game Description
Client
Zeid
Summary
One of Gerwitz's weapons is running amok. You must go to the place where the Crystal Lines converge, use the blood of a powerful Quadav to lure it out, and then defeat the evil weapon that appears.