There are actually three important tips that you can do if you are to make the transition from nightmare to hell in Diablo 3. Read closely:
First, stop leveling the Black Smithing. You can focus on this later but as you progress through the game, make sure you have the right gear that will certainly make you overcome most of your enemies and will not hold you back. To level BS will serve like a gold sink and more over the items you might craft will either just be crap. Save your money instead and buy some off the Auction House. Remember that once you get to hell, you need some great deal of savings. Buy pieces of gear that will help you manage Hell better.
Second, when you have finished gathering up the right gear, focus on your builds. Make yourself efficient when it comes to kiting and be able to take in less damage as you can. In the Hell level, you can absorb some damage from a couple of enemies and still stay healed the entire time. So it is important that you always have some knockback.
Lastly, it can be helpful to find a group if you can. You can finish this course on solo mode but each comes with a weakness of their own as a character. Hence, when you find a group, you can synergize and progress better through the game. These are ways to make everything a lot easier for you to go through hell from the nightmare level.