Beginning and end, both can be quite challenging in writing a novel, at least for me. Every novel has intro, body and conclusion, just like an essay, right?
Well, for me, writing the body is much easier, as thought processes then are freely floating. I usually get stuck in the beginning. Conclusions are easy as by the time you're writing it, you already know in which direction your story is heading. But hey, this just works for me. For you it might be different :)