Don´t the twins just leave to reload after shooting her?
Anyway it should go: Shoot Amy--cover- -dive for cover--cover--dodge left--stall for time--let you off...
And then the rest is a cutscene.
Wait, try to hit, as in you actually clic to do it? I don´t think that is ever an option. When you get to the twins it is just a cutscene where you run out of water and the twins shoot you. Are you sure you are at that part?
That dialog sounds more like what you get when you try to attack the blond and the dual wielding chick. In which case you should not shoot at them and instead take cover behind the tree.
(The gun should be at the shop right after the battle ends with the dialog about buying it. It´s under electronics.)