There are a few reasons why Kakashi is such an adept combatant. First of all, he has a wealth of experience under his belt. Over the course of his lifetime, Kakashi has fought in many battles and encountered a variety of different opponents. This makes him well-equipped to handle any situation that might crop up during combat.
Another big factor in Kakashi's favor is his acute sense of observation. He can effortlessly assess his opponent's movements and determine their next move before they even know it. This allows him to enact quick countermeasures or take advantage of sudden openings in the opponent's defense.
Finally, Kakashi is an highly versatile fighter. His arsenal includes signature ninjutsu techniques as well as powerful guns skills. This gives him the potential to defeat most opponents with a more broad range of attacks.