Be aware of your surroundings (obstacles/map layout) and use it to your advantage. I think this is one of the most important factors in improving your gameplay.
A lot of people complain about how they are dying a lot to a particular gun like a shotgun or handcannon and then they immediately go and say that the gun type is broken before analyzing their own gameplay.
By using your surroundings you die less, can counter gun types properly, win more duels, and escape encounters you know you won't win. Of course your loadout will matter too as that will determine how you would use your surrounding.
If you are dueling and you see that you are losing, get back behind cover and don't challenge again. Find a different position to shoot from or just disengage entirely. People know this, but have a bad habit of not being consistent about it.
A lot of people constantly challenge even when their health is low. Yes you might want the kill and not want to lose the duel, but it's better to heal up and go to a different engagement than challenge with low health.