Major issues I see that really hinder most people ( regardless of class) from doing well.
1. upgrade your gear. If your level 50 wearing level 35 armor/weapons you NEED to upgrade your gear. It matters !
2. Swap your weapons. People run in forcing themself to use just 1 weapon the whole fight. You are limiting your effectivness by NOT swapping.
3. MOVE. Dont stand still.
4. DODGE ! you actually need to dodge incomming attacks.
5. Change up skills for group/solo play. Some skills are great solo, some are great in groups…rarely are they good for both.