This is a story based achievement cannot cannot be missed. You must defeat the boss at the end of chapter 2 (level 2-5) called The Excavator.
I found this to be the easiest of the bosses up to this point. His first two phases consist of a laser turret that attacks with various patterns of laser whilst small flying bots attack from all directions. Use your gatling gun to take out the fliers and then use rockets to hit the turret, whilst doing your best to avoid the lasers.
The final phase is a bit different: a buzzsaw on a long arm will be waving around and you'll see red targets on the ground which will soon have rocks fall upon them. The trick here is the find a place out of range of the buzzsaw and not within a red target, then shoot everything you've got at the buzzsaw. After a certain amount of time, the saw will break and will be replaced with a new one. After you've defeated a total of three buzzsaws, the boss fight will is over.
See the video below for a visual guide (credit goes to PeipeMasta on Youtube). I found the most useful upgrades to be gatling gun and rockets.