Hmmmmm. I cant really say "this part" or "that part", just because it depends on so many facts: in most cases its the skill of the modeler, the scale and the figure itself (shape, details etc.).
Normally I would say its the skin, because thats really tricky and quite hard to learn, especially for beginners. The larger the scale, the more difficult. Thats why I do fantasy/SciFi figures mainly for example - Im really afraid of painting skin. In this special case the figure has no "Pin Up" look and the bare skin parts are not too large and I thought its the armor that really would be tricky - but in fact, its not (thanks to the awesome MrHobby paint!).
Sword in progress:
