This is a solo fight. You do not need to have Geomancer set on main job to participate. Trusts are allowed for the fight, but Fellows are not. Buffs and TP will be reset. Time limit on this fight appears to be 5-10 minutes.
He appears to have a rotating elemental vulnerability but the pattern it follows, if there is a pattern, is currently unknown. Three or four tier 3+ nukes hitting his weakness should be enough to kill him. With the pattern unknown, this is a heavily luck based fight. When you hit a weakness I suggest immediately following up with another nuke of the same element. Often this will also do full damage.