Beat it twice so far, but here's some tips that really helped us through it.

-Positioning of the Tank and Mage party is Vital. Like Coincounter there is an ideal distance where you don't want to be too close because of Leg Sweep but not too far away to avoid that Drain move.
-Save for Meteors and the Drain move this fight is mostly Physical Damage so the tank should be carefully cycling his moves to maximize mitigation. (IE: Aegis Boon > Wears > Divine Veil > Wears > Sentinel > Wears > Repeat)
-Bards should be doing Sacred Prism > Stoneskin everytime a party member/s get hit from being a little too close to leg sweep to help minimize damage on the party. If Sacred Prism is down Stoneskin the Tank and have one of the WHM's Sacred Prism > Stoneskin the Party.
-About 11-15% in the second fight the Bard should pop Battle Voice and Ballad the entire group (Including those up Stairs) before Nael Van Darnus warps to do his Meteor Spam. This helps ensure everyone has ballad for the entire Meteor Phase and helps ensure they don't run out at critical moments.
-During the Meteor Phase the WHM's should be sleeping the Golems and assisting with Nuking the Meteors when they can. The Tank and DD's will not need healing for this part and it's essential that all the Meteors are taken out.
-To add the Tank should be full on DPSing on Nael Van Darnus during The Meteor Phase, ignoring hate management abilities because your trying to keep him from massively regening when it's just you and maybe one or two DD's. (Goring and Riot Blade are wonderful for this part considering he doesn't change position and the debuffs help keep damage up and his hp down)
-After Nael Van Darnus's Meteor Phase if your PLD using Hallowed Ground as a precaution after you reach the bottom of the stairs and pulling him gives your healers a little breathing room considering you might miss Benediction and it will take a moment for the mages to heal you.

Hope that helps.