If you can go back and buy the Semi Auto Rifle.
As for the Garrador try one of these.
1.If you have one hit him with a rocket, but make sure it hit's him, and preferably in the back.
2.Shoot the bells with the handgun, he should run to it and smash it, sticking his claw into the walls at the same time, while he's stuck blast the **** out of his back with any guns you have, make sure you hit the plagas and unload any grenades on him, Incendinary work the best as they stun him to.
But never throw a flash grenade near you or he'll be able to find you, throw it as far away from you as possible and he'll run off towards it, giving you a good chance to shoot his back.
Credit to .Impact for the sig & avy