Which level of writing is the most difficult for you, developing each chapter, scene, paragraph, or sentence? Each level is different in complexity and importance.
A chapter needs to move the plot, develop the characters, and be entertaining. Smaller parts do the same thing, but each chapter as a whole is really about these things, without zooming in on the other areas. A weak chapter concept may lead to the need to prune. Sub-plots are generally manifested as chapter ideas, although they can be of smaller scope.
Scenes take the goals of a chapter and accomplish them. Characters need to be hand-picked for each scene (if you have enough of them to have to worry about it). Scenes are the second-hardest part for me, as I often ad-lib them and usually don't even think of my writing as broken down into scenes.
Paragraphs are the most difficult part for me. I only outline the chapter and sometimes the scenes, so deciding where the story should go within the confines of the chapter's overall plot takes a lot of work. I often worry that the paragraphs of my novel are too short, as I'm writing epic fantasy (generally massive paragraphs), and I even went to the trouble of doing special formatting on my Word document so that the pages look just like the pages of mass-market paperback Wheel of Time books. The paragraphs are what makes my writing pace a crawl.
Sentences aren't too difficult for me, although they may be for others, especially outliners. Weak individual sentences in the middle of a book can be forgiven, but at the beginning they need to be strong. In my opinion, the best way to accomplish this is to let the sentences flow as they come into your head, yet sculpt them to your voice and add some flourishes as well. For example, you may be thinking "Amelia picked up a spear from the ground, looked up, smiled and threw it at the bandit", but it's better as "Amelia scrambled for a spear on the sawdust-laden floor. She sprung upright with it balanced on her shoulder, thrust it forward, and smiled maniacally at the invader as it plunged into his torso." Of course, sentences need to be edited later on (I did some editing on my example).