This is a minigame tha can be beaten in a matter of a few seconds, thats not that point. The goal is to survive as long as you can on each map or round, and when you are overrun move on to the next map or round. If you kill everyone, then move on. Have fun, don't worry about timing or any of that. Survive the zombies! Also note leaving the base spawns a bunch of bots, so don't leave!