Notice, that there are differences between the classes when it comes to behaving on a battlefield, mobility and overall character of their gameplay. If you don't try to hurt your enemies from distance - most of the powerful projectiles and rockets can be avoided when they are flying towards you by moving from their path or, sometimes, even shooting them down. Melee attacks are good against weaker enemies. But what should you do when your plan won't go the way you wanted? The best way is to jump back and hide behind a cover. However, each of the classes can behave differently in a situation like that: Titan can use their barrier, Hunter has Dodge ability and Warlock will most likely use a melee attack which in default can push enemies back.
Remember that your health regenerates after a short while and because of that it is sometimes worth to hide in a place hard to reach and get back a little bit of strength. During matches against other players doing that is easier because when you stop shooting while your companions keep firing then usually enemy will focus their attention on them instead of you.