God help me for necroing this...
I know a billion methods have been looked over...and I checked through this thread as well as BG, but didn't see it mentioned there nor here...
Seems too obvious to even assume such, but just thought I'd bring it up - maybe he needs to be hit with the weapons in a specific order? The final boss in 7th Saga (an older Enix game) required he have the 7 runes used on him in a specific order...seems a bit too obvious that maybe the Virtue weapons hit him in a certain order, but I haven't seen anyone mention that being tried, somehow. I'm figuring maybe it'd based on the virtue numbers or something, but dunno...just a thought.
Sorry if this was already mentioned - I looked thru several threads and didn't see it ever brought up.