Lemme' start out by saying I got the idea from CowThing's Add-On so give that guy some thanks too.
For every NPC you spawn this will set said NPC's difficulty to what ever level you wish. The default level is Legendary. There are five levels as follows:
Use the ConCommand "npc_difficulty <level>" to set the difficult, it will be saved for the next time you play.
Note: This set's the difficulty for all NPC's spawned that use weapons. So Rebels and Combine will be on the same level.
Another Note: Spawn an NPC then change the difficulty then spawn another and each will have different difficulty levels.