I can take these pretty easily as RDM/NIN without Utsusemi Ni. Took about 5 kills to get Illuminink, I basically camped on the stairs leading up to the fountain so I could zone to Hall of the Gods if I needed to, and I didn't get magic aggro either.
These things also drop "Bottle of Illuminink" which is unsellable at AH or at NPC's. It *is* tradable however, and I can't seem to find any information on it whatsoever.
The illuminink and a piece of parchment is necessary to gain access to the Divine Might battle in sky. You fight the battle in an alliance against all 5 of the Ark Angels.