"btw, I got the Dark Knight Dressphere before i fought the guy, so i didn't have a chance to utlize the girl's skills as Dark Knights. "
All you need is Darkness.

"Should i have Yuna and Paine both be Dark Knights? Or should one of them be a Gun Mage and the other a Dark Knight? "
Do what you will. You can always just sphere change in battle if you're not happy.

"If i follow that pattern you have, SeeDRanKLou, How will i be able to have the guy do the Drill Shot at the Gun Mage?

Um...When you said "heal", did you mean have the White Mage do "Pray" and raise all the party's HP? Or "Cure"?"
When I say "heal", I mean use something curative on Baralai. And to make sure the Gun Mage gets the Drill Shot, sphere change whatever character to Gun Mage and have her do the last healing, and she will get the Drill Shot.