You will first arrive here via an elevator in a room on the Fourth Tier. Like the Fifth Tier, there are two distinct halves to the Sixth Tier. When you first arrive, there are only two rooms open. The first has the Menhirrim for Mission 25, while the second has a Menhirrim that will adjust the layout of the tower, giving you access to a third room. This room has stairs leading down to the Fifth Tier, as well as the mark for Mission 25. You will access the other side of this tier by riding the main elevator once it can reach this high. Watch out for the trio of Vampires in the room with the elevator to the apex, as their Aeroga ability in tandem can make short work of your team.