Levels don't really mean a lot directly, it's all about damage output (upgrade weapons) vs HP.
Weapon parts are extremely valuable. Break down shit weapons (like the two non-silenced starter pistols). Move weapon mods around so it makes sense (i.e +crit% mods on weapons that does highest crit damage, etc).
Don't fear high level zones. Go in and scout, try to extract all the resources you can without engaging enemies. Some times you can even get at great items without killing anyone.
You should have at least some of knee shot, tree hugger and hog rush, which all can take one or more enemies out of the fight for atleast one round. Lock them down, set them on fire, fall back, flank, don't take sub-75% shots.
If you're playing on normal and have access to three silenced weapons upgraded one step (3*30 weapon parts), an ambush should be able to do at least 30 HP of damage before the enemy even gets one action. (6+6+hog rush+6+6+6, roughly speaking. Maybe some 5s in there, I'm going by memory). If you know you have the damage and you've isolated an enemy, you don't have to care about cover. You move in for 100% chance shots!
If you're on a harder difficulty, know that kills and 'XP' are squad-level, so it doesn't matter who gets kills for the purpose of reseting cooldowns and gaining skill points.
Also, high cover (75%) is much more valuable than low cover (25%).
