I'm not a high level player, but I've found a couple of things helpful. 1. Don't let trash mobs accumulate by focusing all your attention on the boss. You can get quickly overwhelmed. 2. Take your time. I've found that I get a bit greedy and that can be punishing if I get caught away from cover. 3. Prioritize! Some mobs are more dangerous to others. Some boss fights can go from nearly impossible to almost manageable if you focus on culling certain mob types whenever they appear. 4. Switch weapons! Don't be afraid to switch to a heavy weapon for vehicle or larger enemies.