(SPOILER)Well, I beat both Demon Walls (the second was alot easier than the first), and I basically used two Quickening Chains (one with Ashe, one starting with Vaan), then cast Berserk on Basch and Vossler so they attack nonstop (and Basch's new awesome hammer did alot of damage), and finally used a Reflectga Node. Ashe still got sent to the X-Zone, but I won.

Unfortunately, I didn't have any Gambits for 'Party Member Status: Blind' yet, so I couldn't use the second half of your strategy, but I still won.