Cujo is the first boss introduced to the game and the first enemy that was intended by the developers to be killed. Though other enemies in the game can be killed, it is often difficult without exploits or wasting precious resources. In order to proceed to the actual tower, Cujo must be killed.
Cujo is referenced in the Dog Breeding Report as being the "alpha".
The dog has one attack, which is to bite Daphne. This attack deals moderate damage. Cujo can also perform a lunge, which does not appear to inflict any additional damage. It only facilitates the bite attack. Before lunging, Cujo will stop moving and gather himself for the leap.
While it is wise to craft a couple of healing items prior to dropping into the arena, the area has several cans of food which will be automatically placed into an open weapon slot if there is one available.
The goal of the fight is to stand in front of the electric fences and allow Cujo to lunge. Daphne can then dodge, allowing Cujo to make contact with the fence and be electrocuted. This must be repeated several times before the dog finally goes down.
The electric fences will short out after Cujo makes contact with them. They can be reactivated by throwing the breakers in the alcove area of the arena. Each switch controls one fence. Several severed arms are strewn about the area. These can be picked up and wielded as a weapon. However, striking the dog results in him taking the arm and spending a few seconds chewing on it. This can be useful to distract him while Daphne is turning the fences back on or if she just needs to heal or catch her breath.
The fight can be difficult as Daphne must be standing close enough to the fence to allow Cujo to make contact after the leap, but she must risk being electrocuted herself. Daphne will be killed instantly upon contact with a live fence. Daphne has no dodge, so it must be timed so that she can move out of the way safely but not so soon that Cujo turns to face her or lands out of range of the fence after his lunge. He will not make contact with a fence except during a lunge.
The dog appears to have been conditioned for violence and aggression. This appears to be solely for the purpose of the Trials, as there is no reward for defeating him. His purpose is to prevent access to the physical High Tower. At this time, it is unknown whether other aspirants have completed the trials by killing a "Cujo". The dog could have been placed there specifically for the current group of aspirants to either defeat or find a way to avoid. However, there is no way to avoid him in the game. The left side of the arena is blocked by stone and the right side is blocked by a fence.
Cujo's name is most likely inspired by the 1981 novel of the same name by Stephen King. It could also be inspired by the 1983 movie based on the book. The name "Cujo" has become synonymous with vicious or aggressive dogs. The Cujo from the novel is aggressive due to rabies infection, whereas the movie version makes no references to the disease. Die Young's Cujo is not known to be rabid and has most likely been conditioned for aggression.