Iron Brigade

This one really is actually pretty easy. Play through the campaign until you reach the level Oasis. Use a chassis that has 3 slots on one side, then equip an artillery cannon (I was using "The Pigeon") Then, start the mission, and like usual, Vlad unveils a new tube, this time called the knob. They are small spiky balls that roll in groups and attack your turrets. Simply aim at a group of them (they usually come in packs of at least 5) and aim for about the center to ensure you hit them all.

This is an easy one to pick up after you are introduced to knobs (Arrive in Africa), the easiest level to get this is in the first African level "Oasis". They come rolling down the hill in large groups ranging from 5-10 most of the time. It would be easiest to attach an Artillery Cannon, preferably a stronger one, which you will unlock farther in the game as you level up. Each cannon takes 3 slots, so you will need a better core on your trench than what you start the game with. You start the game and wait for Vlad to introduce knobs, as soon as they come get as close as you can and aim in the center of them all and fire! Bam! One less achievement to earn in Trenched.

On the level Oasis, the second level in Africa make sure you have a grenade launcher attached. Make sure you do not have to many gun turrets because they will kill the knobs. When the knobs are coming down the hill fire the grenades and if you get at least five the achievement will unlock