1. always keep moving, never stop running. when enemy is firing at you move left and right quickly so the enemy can have trouble aiming to hurt you.
2. stay out of huge attacks like 2 soldiers or 2 heavys, if you do want to ambush the attack find a way to sneak up on them from behind and spam mouse 1.
3. when attacking a enemy and you feel like your about to lose run away, remember you are the faster character in the game you have time to run away get health and find them sneak up on them and fire.
4. I will recommend the scatter gun, the force of nature is a situational weapon. When you are using the scatter gun quickly go right in the enemy's face, shoot and quickly run backwards. firing up close with the scatter gun does more damage.