The very first boss that you face in the game, in the second level, is Arkady Orumov.  You don't have to kill him here, and you can't really kill him.  If you shoot him enough, he runs away, and disappears near the beginning of the level.  If you are going to battle him, back away, and once he shoots Alec and "kills" him, use your KF7 from a distance and shoot him in the head.  Be careful, his DD44 is very accurate and can kill you from a distance.

That's not all of Mr. Orumov that you see, of course.  A few levels later, you will encounter him again, and this time, you pretty much have to fight him.  Again, he won't die, he will just run away once he has taken so much damage.  This battle is hard because there is no room to move around, to avoid his shots, and there are a lot of other guys in the place, that will all be shooting you.  I found that if you are lucky enough to collect a grenade during the mission, it REALLY comes in handy right here.  Just toss it right down the hall, and Orumov will be dead.  It will clear out most of the other guys here too.
The very last time you encounter Orumov, you finally kill him.  But it's not that satisfying.  A single shot can kill the general here.  But he is holding Natalya at gunpoint, so yo don't have much of a choice.  Just take a good shot from outside of the room, and he won't even see you coming.  You can finally pick up his DD44, which he failed to drop on the last two times you saw him.
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Alec Trevelyan, your friend for the first coupe of missions.  In the last normal level , the Cradle, you have two different fights with him.  The first is chasing him around the level.  This part is a little tricky, because there will be a lot of other guards around here, all trying to kill you.

The last battle with Alec is pretty simple.  Blast him off the platform.
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For this battle, you don't need any special strategy, just a quick trigger finger.  As soon as  you land on the platform, open up with both of your ZMGs.
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You will hit him, and the force of the shots will throw him right off of the platform.  then yuo will view a quick cut scene, and the mission will be a success!

Xenia is one of the more difficult bosses.  Not because you are in a confined spaced, but because she carries two very deadly weapons.  An RC-P90 and a Grenade Launcher.  the RC-P90 can shred you pretty fast, and the Grenade Launcher will fry you with a single direct hit.  Ine strategy is to plant your remote miens on the bridge, and detonate them as she goes across.  Another strategy is to have Natalya kill Xenia with her Cougar Magnum, while you distract her fire.  Natalya's MAgnum can kill Xenia pretty fast, ina few shots.

Baron Samedi is another difficult boss.  You have to kill him THREE TIMES!  And even then, he isn't really dead.  For each time, try to sneak up on him, and get just enough of him in view so that you can see him, and shoot him in the head, but he can't see you and kill you with his double DD44s, ZMGs, or Moonraker Lasers.  The last battle is very difficult.  Those laser shots can kill you in only a few shots, so be careful.  Once he dies, you view the end sequence, but he is still alive!  But wait!  You can killh im a fourth and final time!  Simply change you control configuration to Domino, and when you are viewing the end sequence, press Fire on the second controller!  Baron will fall over dead!

The last boss of the game, Jaws is certainly not the least.  He stands about 8 feet tall, and holds a AR33 in each hand like they were pistols.  This is an extremely difficult battle.  Jaws takes a lot of hits before he dies!  Your best bet is to get a grenade, and toss it at him from behind, then hit him in the head with AR33 fire untli he dies.  Another strategy is to go in   very close with only your slapper.  If you are extremely close, Jaws can't hit you with his double AR33s.  It takes a while, but you can slap him to death.