Skill accounts for a lot of it, but sometimes a solo instance has a fail condition that isnt as obvious as you would like or a mechanic that isnt as intuitive as it could be. The first time i tried this i had little trouble with the NPCs, but i couldnt figure out how to charge the sword for the life of me and eventually failed. Turns out i was clicking on the wrong thing. When i figured out what i was doing wrong on a second try i had no issue whatsoever finishing.