Mirror Images/Dancer

Did this with DNC/NIN just kept shadows up and used curing waltz 3 when he did blighted gloom, and occasionally stunned. Had fellow go healing style so he could enfeeb and cure himself if he pulled hate.
Ok same thing pretty much. I went in as DNC/NIN, used an Icarus Wing for extra TP, and put up shadows. I ate a piece of Tuna Sushi (though I'm sure I would have been just fine without it)for extra ACC. I had my NPC set on "Soothing Healer".
I went in and started whacking away, gaining steps when available. When Vassago used "Blighted Gloom", I ran and hid behind my NPC. After Gloom, I would use Animated Flourish to gain back hate. I didn't really have to use above Curing Waltz II, except the first time I was in range of Gloom. I also healed my NPC with II after it gained hate because of "Blessed Radiance".
Easy sleezy, I never used a WS, and was done in under 7 minutes.
I did this fight as DNC/nin, with no wing and ate a sole sushi. NPC was set as Stalwart Shield, and when the demon did Blighted Gloom I just ran behind my NPC if I had hate. I did a few Curing Waltz III to my NPC, but overall, the fight was a piece of cake!