You can't please everybody, but you'll satisfy most of your players with a medium challenge level. If your mission is for a small circle of friends, tailor it to their expertise. However you go, use the mission's text file to rate its intended difficulty. Don't Compete The most common mistake of beginning mission designers is to think that they're competing with their players. No one likes a mission that hammers them with every negative event that you can script.
Balance is important.
Time to recover can be critical. It's easy to create an unwinnable mission, and no one will respect you for doing so. Think of your mission as a story that you're telling to thousands of strangers all around the world. Thrill them, surprise them, guide them, reward them. Don't try to kill them.