Unlimited is a game feature where the Survivor is placed in the Lobby area to kill as much as zombie as he/she can. Zombies start swarming from one direction, until they just appear in all corners as the survivor continues to do anything he/she can to survive the swarm that is coming for him/her.
Zombie dogs also appear to get things interesting, and the same mechanics still apply when a survivor provokes a zombie, it runs. Using the bullets that has been bought before entering the Unlimited, be warned that it gets used up here as well, and will not replenish.
But all is worth it with the gold that is gained by the numbers of zombies killed during the Unlimited. The more the zombies, the greater the rewards.
Aim for the head and get as much headshots to gain more ground and space between upcoming zombies.
Make sure to turn to both ways, and listen for a bark since dog zombie are smaller and will be able to slip past human zombies.
When death comes, watch an ad and revive! Spending 1 or 2 gold would also be worth it since the reward gold in Unlimited will make up for the golds you lost.