Mirror Images/Corsair

COR/RNG - This is pretty much impossible without massive amounts of hi-potions. Even then, it may not be successful. Tried it twice with NPC level 50 set to Stalwart Shield. 1-2 ranged attacks would steal hate, and just a few hits from Vassago was enough to end the fight. I did not try this with a healer combat style, but I can't imagine it going much better at all.

COR/WHM - Standing around curing your NPC while they tank is the way to go. On my first two attempts, I just stood around curing my NPC while she tanked. In the end, we both ran out of MP and Vassago wasn't even at 50% yet. On my third run I tried a new strategy. This was done with a non-merited COR and a basic MP+ set-up (with some AGI boosting gear equal to +10). The rolls I used were Evoker's and Ninja's. When I went in, I put up Evoker's roll, buffed with protectra/shellra, put up Dia, Blink (on myself) and Regen (on NPC), and let my NPC tank for a little while. When I was certain her timer was up, I would Ice Shot and RUN. It was Iceday and my Ice Shot was doing 200+ damage with each use, so I would instantly have hate. Adventuring Fellow would provoke as soon as possible, and I would go back to buffing/debuffing/curing when needed. I'd constantly watch for Hecatomb Wave so I could cure my NPC's blindness. It's important to run in circles around the arena after landing Ice Shot. Your NPC will eventually provoke the demon back. Also, keeping BLINK up is extremely important. Vassago hits very hard. It shouldn't take too long to wear him down. My fight lasted a total of 8 minutes, 59 seconds. To ensure victory, you may want to bring a Yagudo Drink or two. I managed to win without resorting to that. I DID bring Rolanberry Pies, however.