This is easiest in a long game of Gardens and Graveyards. Make sure to setup near the objective and make sure not to waste shots when enemies are far away and can easily get into cover.
Another good place to setup is along the path to the Garden where you can setup behind cover and Zombies will walk past you along the way. This allows you to stay safe (especially from ZPGs) and lets you get the jump on enemies.
You can get this fairly easily if you are in a game where the zombies take over a few Gardens.