Mark I always liked the idea of the gunblade. It's not meant to shoot at a distance. When you slash someone and pull the trigger at the same time, it parks an explosive quarrel in their chest. I thought it was an ingenious way of having a sword that can defeat armour.
Only party members that can swim, and they have the most powerful limits.
Send Rikku down Aurons path to start, she will quickly become your heavy hitter. Or, send her backwards down Yuna and Lulu's paths to pick up their level white and black magics, then tackle Aurons path.
Really, every character has utility, and the only ones you are forced to use are those three.
