In the Challenge mode, the sim names are based on their characters (so a ShockSim is the Shock Trooper, a BlondeSim is Mr. Blonde, etc.). You can check what difficulty they are on after you play a challenge; the options will be whatever the ruleset is for the challenge, so you can go to the Simulants menu to check what difficulty it is.
The CPU tend to pick tall characters if it's their choice. Tall characters move faster, and I would not be surprised if they get move headshots on you because of the extra height. You've got to remember that this is the Nintendo 64 we're talking about, so the A.I. script is pretty simplistic, assuming they didn't change anything for this release. The A.I. doesn't really account for what would be human error, so on any difficulty setting they have pretty good accuracy. Their accuracy also doesn't seem to decrease at long range. The easier sims account for this by pausing before they fire, as well as moving slowly. They also don't look for traps, so explosives are a good way to handle them because they will walk right into them. Their movement patterns are simple, they will either stand in place firing at you, or move along their paths through the level if they are going for a shield/weapon. Knowing their behavior is really the only way to beat them.
I could never beat anything better than a NormalSim myself. I had to beat the challenges with 3 other players who were actually good.