Defeat the giant spider.
The Giant Spider is the mid-boss of the first zone. To kill it, shoot it in it's weak spots with the flak cannon as much as possible, and switch to your other weapons to kill the smaller spiders that emerge from the hole. Be wary of walking directly in front of the spider as it will charge you and blow up your vehicle, leaving you vulnerable and under-powered. This achievement can be unlocked on any difficulty, and will unlock when you finish your current zone, NOT when you meet the requirement.