There are many outstanding monsters and aliens in the Ultraman series, but the best monsters or aliens may vary from person to person. Here are some of the monsters or aliens that I think are well-shaped:
1. Jayden: Jayden is a very classic monster in the Ultraman series. Its image is very realistic and full of power and mystery. His attacks were also very diverse and very brave in the face of Ultraman.
2. Giant snake beast: Giant snake beast is another classic monster in the Ultraman series. Its image is very powerful. It looks like a huge poisonous snake with strong attack and defense ability.
Vladi: Vladi is an alien creature in Ultraman. Its image is very cool, with a black shell and powerful energy output. His attacks were also very diverse and very cunning when facing Ultraman.
4. The arcturus: the arcturus is another alien creature in the Ultraman series. It looks very mysterious and looks like a huge star with powerful energy and defense.
Vegeta: Vegeta is a superhero in the Ultraman series. He is an alien and has powerful combat abilities. His style was also very unique, full of technology and personality.
The above is just my personal opinion. Different people may have different opinions.
Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.
Perhaps in this fanfic, Griffith has turned into something truly evil and a threat to the world Guts cares about. Guts, being the warrior he is, might feel it's his duty to put an end to Griffith to save others. Also, Guts could be driven by his own sense of justice. If Griffith has wronged innocent people, Guts may decide that killing him is the only way to set things right.
It depends on the specific fanfic. Some fanfics might have Guts killing Griffith as a major plot point, while others could take a different direction and not have that happen at all.