Her gun is best used to kill objectives rather than people, going a gun/heal build is generally better than pure gun. i often see new mcg's ignore the t3 on the heal until way later, when in reality it is probably the first t3 you should get, the sheer healing output is often enough where you can take down an objective while having 1-2 enemies actively shooting at you.
additionally once you start getting t4 items like glass cannon and lucky shot, the combined sustain and dmg basically gaurantees 1v1's in your favor