I picked Koloktos for one major reason. let me back up a minute though. I didn't find any of the bosses particularity difficult, once you knew what to do. Figuring out what to do however, was a challenge with a few of the bosses. Scaldera specifically for me. I felt stupid after I figured it out, but it took me several tries. My point is, after you figured out what to do for each of the bosses, they became very easy. Dodge, hit, dodge hit. Koloktos on the other hand, did challenge me in the implementation of the 'what you have to do'. Meaning, it was obvious about the hands, legs, and glowing red orb, but i did have trouble controlling the giant knife. For the reason, I picked him as the most difficult.