I did this in a private match solo on Dead Rock - this helps because nobody can steal your kill. You need to survive until wave 5 for a mini boss (big guy with a chain gun) to appear.
I played through the match as usual, keeping to the railings near the spawn point (cliff drop behind me, facing the tornado). For most of the enemies, you can kick/leash them over the railings for easy kills. When you're swamped with melee enemies, go up the nearby steps up with the dead end (un-openable door at the top left of it), keep your back to the wall and kick enemies into the generator.
By the time wave 5 comes, go to a drop point and buy a thumper and a couple of PMC charges. This wave doesn't consist of solely a mini boss, so stay alive whilst looking out for him. When you see him, arm your charge and shoot him One charge should be enough to stun him, if not, unload another into him. As soon as he's stunned, unleash the thumper and the boss will die, earning you the achievement.