Ok so you have Quistis in your party so things will be pretty easy from here:

For the Malboros; make sure Quistis is with yellow life and that she as the initiative abiltiy then use degenerator and Malboro will be defeated withought having time to use Bad Breath.

The same rule can be used for the Ruby Dragons but the Ruby Dargons that you fight before facing Bahamut cannot be degenerated (at least i think they canīt) so make sure someone as the initiative abilitie and call a GF so if the Ruby Dragon uses his Dragon Breath atack the GF will die for you and you can then revive the other two characters, now i always have Zell in my party so he can use Armageddon Fist to get rid of the Ruby Dragon or use Irvines Shot limit break, you could also use Squall but since Lion Heart is random i wouldnīt recomend it.

And thatīs the bottom line