Another good question I just thought of is do you play on Speed 1 when you're doing this? If not that can make a pretty significant change in keeping the debuffs up due to things like input lag.

But yeah you gotta figure if you're doing 4 SBs in a row with Fran like that then something is probably going to fall off in the mean time. I think it might be better to try starting with the SBs first and then redoing the Armor Breakdown and Full Break. Since Vaan is also instant cast, you can wait until closer to the time you're ready to do the big hit to refresh his entry.

Yeah I do think overcharging Raijin by one turn might be needed to hit maximum damage. It allows you to get Armor Breakdown or Full Break (maybe both) debuffs back up on the boss before the big hit.

Remember that Shout does not stack with Meteor Crush. Ramza's USB also is definitely a huuuuge help with those instant party actions. If all you're doing with Ramza is his USB and Chant then consider doing USB twice then Chant if able. Those two instant casts will drastically speed up your party during the SB casting steps. That should allow Fran to get out her Imperils and the Penta Break plus refreshing Full Break all in a much shorter period of time so that debuffs are less likely to fall off. Vaan takes care of Armor Breakdown and his -MAG/-DEF entry at the same time. Give those suggestions a try and see if they work for you. :P

Also, I just remembered! Raijin's commands are Monk school so make sure to equip him with the Monk +40% RM instead of just a Weakness +30% RM! :D