I play solo so I'm pretty confident that I at least somewhat know what I'm doing. Hazard 4 has become my chill "I don't have to try too hard" difficulty that I play all the time and basically never fail.

The game isn't about killing bugs, you won't get a lot better at it over time. Don't focus too much on it. (You can min max ammo vs damage all day but it's less important than finishing the mission long before you are overwhelmed).

The game is about doing the objectives efficiently. You will get faster at finishing missions, which means you will fight and kill less enemies during the mission. This is by design, the gold and XP you get specifically for killing enemies at the end is worth much, much less than even the secondary objective.