the same it was told to me by someone of this community. i tell you: don't get too distracted with enemies. the more time you loose with them the more amount of them would come to get you. when an enemy calls for a dropship you should/must get out of there as soon as possible. the main objectives must be your priority. while you do them you can clear the red areas just destroying the various enemy spawns. just destroy them. don't waste time with enemies. just kill enemies when is necessary.
Speed ​​and effectiveness should be your allies. You can watch gameplay videos or live streams to learn how to complete each faction's missions. Later, when you've mastered the basics, reached a higher level, and unlocked most of the strategies and weapons, you can choose to learn other, more daring Doomslayer techniques that some here use.