It won't admit it, but the computer isn't very smart.  Sure, if you are on 00Agent and 5 guys are firing at you, they can probably kill you, but if it is you and another guard, mono y mono, you will definetly win.  So here's some strategy for taking out multiple guards.

Stay behind them!
I can't stress this enough.  The guards will open fire on you if you are within their 180 degree field of vision.  Stay behind them, and give them a good solid chop to the head.  Don't bother wasting ammo on them if you can.

Back up!
Remember that the computer doesn't have a choice of weapons.  So in most of the early missions, the guards have KF7s or Klobbs.  These are pretty inaccurate weapons, so if you are getting shot at, and you are at medium range, back up!  Don't turn and run, just go backwards, firing all the way.  If you are using your PP7, which is a very accurate gun, this will work pretty good.  Don't attempt this if the guys have really fast or really accurate guns, like the ZMG, RC-P90, or AR33.

Duck Behind a Corner!
The Guards have pretty slow reflexes.  So if you are getting shot at, go behind something.  Then wait for the shooting to stop.  Come out from behind your corner, and start shooting.  The guys will be running towards you, and you can mow them down with a hailstorm of bullets.

Strafe Around Them!
Once a guard starts shooting, they pretty much can't move until they stop.  They can move a little from sode to side, but not much.  If a guy gets down on one knee and fires, he's toast.  Strafe around him, so you are at his side!  Simple!  Karate chop him to save ammo, or get some clean head shots to take him down fast.

Get Them From a Distance!
This is a good strategy if you are facing lotsa guards.  Guards only shoot if they are within a certain range.  So when they open fire, simply move out of that range.  They will start running towards you, to get into that range.  When they are out of their range, but in your range is your opportunity to put holes in them.

Line em Up and Blow em Away!
Keepin mind thatsome weapons, like the AR33 or RC-p90 can shoot through one guard into the next one.  Like in the Train on Agent, with that RC-p90, you can be hitting 5 guys at one with a single bullet.  This really helps your accuracy too.  Soifyouare facing multiple opponents, it won't hurt to shoot more thenone of them at a time.

Use Drone Guns to Your Advantage!
Drone guns can kill guards.  So, try to line up some guards with a drone gun.  You can't do this in many places, because there aren't a lot of drone guns.  This is a good strategy in Jungle.