Hmm, I used Commando as Lightning's main role, Sentinel for Fang. Do you have Phoenix Downs to use for quick raising when Lightning gets KO'd? That may help too. If she's your lead character and she's the only one who gets KO'd by Destrudo, definitely switch to someone else for the battle. Fang pretty much never gets KO'd as long as she's got Mediguard up.

~DragonHeart~