So the irresponsible kappa girl gets hired for a mission where they defend a fortress from dragons, upon the arrival of the invaders, her initial action is to goof off and take a selfie(with their high technology, their smartphone equivalent devices can actually hover hands free) just moments before the invaders attack. Of course, her high rating is due to immensely powerful defensive skills with that small shield/buckler of hers. To put it in simple terms, if she where a pokemon, she would learn moves like Mirror Coat, Metal Burst, and Counter, as well as defensive techniques like King's Shield and Protect. That's the sort of fighting style she has. She has to pick the right attack counters,(such as for physical or energy based attacks), as well as the fact that she has different varieties, such as a reflect type technique that unleashes an explosion in response, or uses the attack energy and shoots it back as an energy beam.
The gunslinger girl is from the same organization as the other gunslingers. They all use the same style of fighting, which is too manifest/materialize/conjure up bullets for their guns(and all use the same naming convention, but how the bullet is made differs from gunslinger to gunslinger, as some use solid bullets, while others will even use telekinetic bullets made of pure psi.