Rule #1: Always match elements(or at least have a plan to deal with shields/armor).
Rule #2: Always be aware of you surroundings. Use cover. Never stand in the open where multiple enemies can crossfire you.
Be mindful of where you enter an area(rule 2) from and what areas are vacant of enemies. Use these locations to strategically fall back to. Set up ambushes in your mind and then kite enemies to that location for ambush.
Whenever possible, lock multiple enemies up at choke points.
Use terrain and elevation to your advantage. Be aware of horizontal and vertical axis in relation to enemy position and use this to your advantage(rule 2).
Rule #3: Be unpredictable. Stay mobile. Always bob and weave, left and right, back and forth ,stay in motion. Circle/strafing enemies is great but don’t fall into predictable patterns.
Use enemies as cover to block enemies behind them. This goes back to the the rule #2 as well. Keep track of enemies locations as they are moving so they don’t surprise you or put you in crossfire.
Use everything to your advantage.