The combat simulator is the fancy name for Multi Player.  Almost every serious Perfect Dark player has their character saved, and works on building that character's statistics.  The most important of those statistics is not how many kills you have, or your accuracy, but your rank.  Listed below are the different ranks you can obtain.  The list of ranks was provided my Sharkman.
 

21 Beginner  20 Trainee  19 Amateur  18 Rookie  17 Novice  16 Trooper  15 Agent

14 Star Agent  13 Special Agent  12 Expert  11 Veteran  10 Professional  9 Dangerous  8 Deadly

7 Killer  6 Assassin  5 Lethal  4 Elite  3 Invincible  2 Near Perfect  1 Perfect

A perfect rating is near impossible to obtain.  If you are able to get it, you are indeed a perfect player.  At this time, the ranking system is not fully understood.  We don't know exactly how you advance from rank to rank.  Somebody will think they have figured it out, then somebody comes along and proves them wrong.  So all we know is that you kill players and get a better rank.  So kill a lot of players!

Now, here is how you can increase your ranking faster.  There are other things that you can do, but we don't really know what they are yet, so here are a few basic ways to get your ranking up.

Get a lot of kills.  Duh.  There is an easy way to do this.  Go to the Facility. (Felicity)  Go to the very end of the level, the dead end.  Make sure that you have a gun in that end room, not a shield.  Having a laptop gun in that room is even better.  If you do, take the laptop gun, set it up as a turret in the door way.  It will kill everyone that comes through that hall.  And if you have a gun, you can combine fire with the turret and get kills fast.  If you have the infinite lap top gun turret ammo cheat on, you can just set this up and leave the game running for a few hours, come back and have 1000 kills.

Fight against harder sims.  There is an easy way to kill advanced sims.  One hit kills, or License to Kill for us Goldeneye-ers.  Put on pistols.  Use a big, open level like Temple.  If you have two pistols, you can fire pretty fast.  Having a shield gives you two hits before you die, and the computer only gets a few shots in before you get them with the pistol.  So fight one Hard or Perfect sim, and you won't have a problem.  Try waiting in the main room for the sim to come up the stairs on either side, and crouch down you can shoot them in the legs as they open the door.  Remember, one hit kills.  Even if its to the legs.

Fight against humans.  This hasn't been proven, but playing against other human players seems to help out your ranking more than just playing against simulants.  Even if you just plug the other controllers in and leave them, it will still count it as playing with other humans.

Get lots of medals.  Again, no proof, but it seems to help you out.  So play shorter games.  Play to about 20 or 30 kills, instead of the kill-fest 500 kill games I like to play.  You will get a medal or two for each game you win.  Try to get all the different types of medals.  I have gotten absolutely no accuracy medals yet.  I have 6 head shot medals, and a hundred of the other two types.  So try to balance it out.