Ok, going to give a rundown of every bit I've learned. Much of this has probably already been discussed, but hopefully there will be some new gems for you guys
-He begins in Phase 2 (Nael Deus Darnus)
-He immediately begins a Megaflare phase
-Even with destroying all the meteors, it will still do ~6,800 damage.
-With Sentinel & Stoneskin, it becomes ~3,800 damage.
-With Sentinel, Stoneskin & Sanguine Rite, it becomes ~2,000 damage.
-With Sentinel, Stoneskin, Sanguine Rite & Lunar Curtain: ~1,000 damage.
-Deaspected Clusters drop off of Lunar Fragments not spawned for a Megaflare
-Lunar Fragments not spawned for a Megaflare that touch the ground become Lunar Golems
-A Lunar Fragment touching the ground will do ~1,800 to the entire battlefield.
-There is nothing special up top to mitigate damage
-Megaflare is triggered by doing enough damage to him while he is charging.
-Dalamud Horn does nothing.
-Battle Voice or HP drinks can help survivability
-Drop rate on Deaspected Clusters off of non-megaflare Lunar Fragments isn't too bad.
-He uses a new weaponskill that inflicts Defense Down and gives him Defense Boost, in addition to his old moves.