I target with my mouse. Best way to hit what you want. Otherwise, when the target is blocked, and I have lots of time, I use the left direction pad on my XBOX controller to toggle through targets. The auto-target system is seriously flawed. If you face the target with it set on auto-target enemies only, it usally targets with a "A" click. I have it set so all I have to do is target and fire, no secondary confirmation.