Anyone who can sub Ninja will have a far easier time of this BC. That includes mages, as most of the attacks in the fight are absorbed by Utsusemi.
You may need someone to operate the Yellow door. The closest Monolith to your location is around I/J-9/10.
Go to (I-6) in Ru'Aun Gardens and enter the Shrine. You'll be on Map 2. Sneak and head south. Go past (J-7) through the first yellow door. Go west from here, to the room with 2 Monoliths in (H-7). You'll need Invisible, then go south to a room with 2 Dark Elementals around (H-10).
Go north, down a flight of stairs to the Celestial Nexus at (H-9).
Gather together and get ready for the last BC of the expansion. You'll be fighting Eald'narche, who has two forms.
Go all out, and defeat the Exoplates as quick as possible. You'll want to keep mages far out of range for Phase Shift. When it appears time for a Phase Shift, try to get everyone healed to full and get shadows back up. This will negate most, if not all, damage.
If you wipe, wait until you are ejected, as you'll aggro anywhere on the battlefield.
Vortex: Medium-range AoE, low damage (~100), with an additional effect of Terror and Bind. Absorbed by Utsusemi.
Stellar Burst: AoE, low damage (~100), with an additional effect of Silence. Absorbed by Utsusemi.
Omega Javelin: Single target damage (~150), with an additional effect of Petrification. Absorbed by Utsusemi.
The second form is against Eald'narche only. He will teleport around like Ark Angel TT and cast various Black Magic spells, including Bindga.
He still has relatively low HP (~2,500), but may or may not have a large defense and magic defense boost.
You have 1 minute before he aggro's, regardless of your range from him.
It has been observed that he may be heavily resistant to magic, however, a Tarutaru BLM/NIN was seen doing all of the damage to this form, ending the fight in 3 spells.
He has very fast melee speed, but not quite the speed of Hundred Fists.