1.) Setup-time is included in the 30 seconds, but ignore it at this point. It won't matter.
2.) Select a Level 1 mission and place a landmine at each of the three zombie-entrances, 2-3 spaces in from the edge of the map.
3.) Position some snipers or infantry to mop up any survivors, just in case. (Although the mines will usually kill all zombies.)
4.) Start the mission and quit immediately (because you're already over 30 seconds.) Then simply restart again with the same setup, which allows you to easily finish in under 30 seconds.
PS: The same strategy also works with dynamite, if triggered at the right time.