I go slow, circle the battle field, kinda strafing, shoot the melee enemies first, they can be dangerous if they get close, then just never stand still. And I always use the boom barrels.
I don’t use cover, not yet at least, I just think it makes me slow so fast enemies come too close and bat me to death. Left trigger can be helpful though to shoot over obstacles. I assume that cover will be important later on, the dev spent like 5 minutes explaining how the cover action works. But so far never standing still has been a successful strategy for me.
And choose side missions wisely, I tried to go after a level 25 mission or whatever at level 5. All enemies had red skulls as level indicator. Not a good idea!