To add on to this, the latest highest difficulty content type was added to DMM on August, named Guild Order. It uses the raid event eidolons as its bosses, but gameplay-wise it seems to resemble Advent style. Except it has 5 stages instead of 3 and it's effectively Ragnarok difficulty. For example, the first Guild Order is Crom Cruach; she has 8.5 million HP there (...wait, that's actually higher than her 7 million HP in ragnarok difficulty of her actual raid event). That's also still less than the sum total of grunt/miniboss HP in stages 1-4 :P