Play the game in easy mode first, and complete all the missions. Then wait through the credits and it will give you the option to go to the armory or to continue. Go to the armory and get what you want if you want to, then chose continue. It will ask you if you want to start a new game with your current character. Chose yes, and then start again in normal mode. You will have everything that you ended with in easy mode, and getting through it is a breeze. It will also allow you to get some of the secret doors and such that you were not able to get the first time through.
I would assume that if you finish the game again in normal mode, it will unlock hard mode and you will have the same option to save the current character and go through it the same way.