While playing in Solo mode, get a crawler or a very slow moving zombie at the end of round three. Open the door to your left of where you spawn and to the right of the Pack A Punch machine if you are facing it, then the next door following it as well as the debris from the stairs. Fall off, activate the power, go back, and open the door across the bridge, then activate the teleporter and come back to it. Crouch in the corner directly below the black triangle above and use the Right Analog-stick to turn. Finally, stay in the back corner of the teleporter and wait for the last zombie to appear. Kill it. Once this is done, stay exactly in the same position you are at and the rounds will just skip by. You won't see people but you will hear them. This is the same for dogs. Nothing can find and attack you. By doing this you can make it to round infinity, but once you leave you are on your own. You will have unlocked the achievement by round 20.