Start a solo game with all multiplayer options turned off. Equip Deployable Pyro Minefield as your explosive. This explosive is earned the third time you reach Wave 5.
When the game starts do not repair the damaged wall and instead plant the pyro minefield right on top of the technician and deploy it. This explosive actually damages the technician if the trap sets him on fire. When the wave starts bump into AI's shooting at the zombies (as per SirScatterbrain on the other solution). This will stop the AI from killing the zombies. The zombies will come in and activate the pyro minefield which will damage and eventually kill the technician.
Using this method I got this achievement the first time I tried this method with more than a minute remaining in the wave.