Saying "Git Gud" is not very helpful so no, instead ill try suggesting things which can help.
1. Use line of sight to give you an advantage against ranged enemies. If they cannot see you, they cannot hit you. Hide behind walls, in enclosed rooms, etc and restrict how many enemies you fight at a time. This was how I did the fight you complained about further up, when the casters appeared I just ran into the room, cast grease at the door and then used cantrips like Ray of Frost on them as they walked in.
2. Use height (when its an option) to your advantage. Stand on high ground, shove off any enemies when they come close and then use ranged powers or a bow against them, as you will have advantage.
3. If you see someone hostile to you at a distance and you know moving closer to them will engage combat, enter stealth, position your characters appropriately and then use sneak attacks to initiate combat. This will give you a surprise round which will allow you to alpha strike them.
4. Make use of crowd control effects and appropriate buffs for encounters.
^I agree 100% with this.
