This is a glitch that can come in handy. First, start a mission on expert difficulty. Next, save in the middle of a mission and then exit back to the title screen. Now, change the difficulty to novice. Do not load the save file with your current mission yet. Instead, start a new game, and then do a soft reset (press SELECT + START). Finally, load the save file with your current mission.

Now the game will still think your mission is on the expert difficulty, even though all the enemies will be set to the novice difficulty. This can make getting the 10th and 11th level weapons a lot easier.