User blog comment:JWarrior89/Season 3 Episode 2: HK-47 vs Urdnot Wrex/@comment-5618237-20130122034926

Also, as for combat experience, It really depends in which timeline HK-47 is. Being with Revan yet appearing after the clone wars would put him somewhere around 4000 or 3500 years old. A droid like him doesn't stay out of combat for that long, giving him plenty of time to refine his skills. Unlike other droids, he is quit capable of learning through this time of activity. It all depends on the timeline though.