The Big One is an almost invincible and huge zombie. It is seen in Level 3 zones, i.e., Pine Wood, Limestone Spires, Oak Grove, and Frosty Backwoods. Sometimes appears in Level 2 zones, such as the Pine Grove, Limestone Cliffs and Oak Bushes, when he spawns the ground will shake, indicating that he is charging towards you.
But, the update Beta v.1.8.5, the Witch has replaced this Zombie (The Big One). To find him, go to the Second Floor of the Bunker Alfa. However, it has been trapped in a room, and is blocked by obstacles.
This zombie can be killed, but it only takes 1 point of damage for every 20 you deal to it. You could take 2 full SWAT gear sets, First Aid Kits, and Shotguns, Modified Glocks, VSS Vintorez, or Miniguns. Only weapons with a base damage of 20+ can harm him.
Another idea is to buy packs in the store and then grab guns out of your inbox as you need them. (First Aid Kits and SWAT armor in your inventory.) Tested in Beta v.1.5.6, however, a huge amount of guns is required (7 VSS Vintorez needed to take him down, but the guns are rare).
He is currently not supposed to be killed, but it CAN be. Advice is to run immediately unless attacking on purpose.
Avoiding The Big One
If you are out farming and happen to encounter a Big One, you may try to run him out of the map. Video is provided to show how you may attempt such a feat.
Additionally, you may also run to the closest area near you on the map, and come back, as the map has reset and the Big One should have disappeared.