Goat Simulator 3

After reaching S rank in the Goat Castle (refer to New Goat Order (15G) for doing so), you'll be able to open the large doors in the Castle and head inside.

Inside, you'll see the farmer and he'll tell you about a boss fight. Go up to him and knock him off the edge into the void. Now the boss fight will start. 

The first fight you'll need to avoid the spikes coming out of the ground and wait for him to jump into the center of the arena, then attack him until he retreats. Eventually he'll summon tractors instead, and these are even easier to avoid. Killing hitting him until you drain his health bar. 

The second fight is hide and side. In the first round, he'll be hiding around the edge of the area behind something. Spin the camera to find him, then headbutt him. The second round has a bunch of crates on the ground. Headbutt them until you find the one with him inside. The third round has four ladders around the edge of the arena. Climb up any of them and look around the rafter to find him to headbutt. The fourth round has a ton of clones. Just keep hesdbutting then all until you find the real one. 

The third fight is a horrible old school platformer with terrible controls. You can't kill anything or collect anything. Just head right and avoid the thwomps and saw blades. When you get to a seeming boss, just head right past him and into the castle door to end this fight. 

Lastly, you'll be in a room with the farmer. Simply destroy all the servers to end this phase. 

With all that done, it's time for the Halo-esque escape sequence. Hop in the car provided and drive to the end. Don't worry about the timer, as it's not a real timer. Just drive to the end to win and unlock this achievement.