This is how you win, read carefully:
Left Button is your primary, it is also the first thing you should use. Just make sure you have armor lock equipped at all times, this is critical guys.
Left Trigger is your secondary, grenades are very effective close range. I hasn't managed to use them without killing myself, but I heard there was a way to do it.
Right Trigger is your guns, your guns should be your last resorts because aiming's pretty hard. As long as you armor lock spam you should be fine tho.